I must confess that I am dumbfounded as to how about 40% still think Taim is Demandred, even in the face of overwhelming evidence. The arguments that Taim is Demandred have dwindled to pretty much three points, which I will discuss here. I'm not going to go into the evidence against why Taim is just Taim, because it's well known.
1) Taim Looks Like Demandred Enough for LTT to rant about it
So what? Isn't it possible that throughout the course of history, two people might look just a bit alike? I look enough like my brother for us to be twins, but no one accuses ME of being my brother in disguise. Rand might even look like LTT used to (at least when he was young). It's just coincidence.
2) Taim Hates Rand
Of course he does. For thirty some odd years, Taim has been romping around the world, learning about the Power. About a year or so ago, he declared himself the Dragon Reborn and tore large chunks out of Saldaea. Then, Rand comes out of nowhere and causes Taim's defeat (remember in The Great Hunt/The Dragon Reborn?). Taim then has to run for his life. He hides out with Rand, who sets him up as a glorified baby-sitter for the Asha'Man. He's extremely jealous that a kid from a backwater has achieved more power than Taim ever did. There's no need to accuse him of being Demandred just to find a reason for him to hate Rand.
3) The So-Called Aiel comment
To my knowledge, this has never been explained in any way other than Demandred knew a different Aiel in the AoL. Here is what I think. Taim has been channeling for 30 or so years. He probably started around 18 or so, which makes him about fifty. This means that he was in his thirties when the Aiel invaded. Recall, everyone in Randland (i hate that name) is terrified of the Aiel. During and after the Aiel war, they were depicted as monsters. Bloodthirsy killers who owe allegience to no one. Then, Rand appears with a bunch of Aiel totally under his control. Rand's Aiel do not seem all that bloodthirsty at first glance. It's quite possible that Taim believes Rand gathered an army from somewhere or other and dressed them up as Aiel. For Taim to accept that Rand is strong enough to survive the Aiel waste AND gain the allegience of the Aiel is too much. He just doesn't believe that they are really Aiel. Simple.
There. Let the flaming begin 
Bad Ash says: As someone who still remains a proponent of the Taimandred theory, I must admit that you propose a compelling argument against it. In fact, nearly every piece of evidence that is used to argue for the theory can be used to argue against it, and vice versa. I suppose only time will tell, and hopefully this will be put to rest by book 10. 
Comments
What about all the old knowledge that he has. I guess you could say that he's a scholar of some sort of another but if I recall (sorry that I don't have any quotes to back this up) he sounds as if he actually lived the AoL, and doesn't just happen to know about it. I'm pretty sure that's Taim. Maybe I'm thinking of someone else.
Why are people trying to make something bigger than it has to be? The Forsaken turned against LTT because of jealousy, greed, etc. Taim is strong in the power and has been around a lot longer than Rand but Rand has done more. The Asha'man are going to be important for the Final Battle but Taim won't get the credit because he is not the Dragon. He is becoming like the Forsaken in the Age of Legends. Now, this argument can go either way. Another theory on this site mentions that in Winter's Heart Demandred was attacked by an Asha'man but he didn't recognize him. If he was Taim, he would he would recognized an Asha'man.
...Well I guess until we either A)See the two in the same room, B) One dies (right in front of every body, and most likely with balefire so that you can't argue that they are brought back to life in 3 seconds) and the other is still around or C) RJ includes in book 10 a chapter titled 'TAIM VS. DEMANDRED THEY AINT THE SAME GUY FOR LIGHTS SAKE' There will always be someone clinging to this debunked line of thought.
The horse has been flogged into hamburger folks, let it rest now.
Sooner or later it all connects back to itself.
The Forsaken don't only compete against Rand, they compete against each other. For the most part, there's one Forsaken per nation/organization/etc. Let's do a list, shall we?
For some of these, we assume. For others, the deed is done.
4 have or had ruled nations:
Rahvin - Andor
Sammael - Illian
Be'lal - Tear
Graendal - Arad Doman
4 were "independents:"
Lanfear (Cyndane)
Moridin (Ishy)
Asmodean
Moghdinean
That leaves 5 super-national organizations to infiltrate:
Children of the Light
Seanchan (Semighrage)
White Tower (Mesaana)
Salidar Tower (Aran'gar)
Black Tower (Osan'gar)
We know Moghdinean and Cyndane are under Moridin's direct command, thanks to the cour'souvua. Only one Forsaken is left for the CotL: Demadread. We learn that Valda and other Whitecloaks were hunting the Masema when Amador fell to the Seanchan. Demadread has to be in the CotL, probably as Asunwa, that wacky High Inquisitor.
Note: this list, save a few Forsaken, is conjecture, and should be treated with a pinch of salt and a slap in the face.
Timekeeper
Guardian of the Wheel
--The Creator made women to please the eye and trouble the mind. 
--Kittens tangle your yarn, men tangle your wits, and it's simple as breathing for both. 
Demandred might also be among the dragon sworn helping to sully Rands good name. Of course the only one with any prominence is Masema and I think we can safely say that he hasn't been shang haied and replaced with someone. He was getting a little bats assed crazy when he was last with Rand in the begginning of TDR. If that had been D-Man then he'd have tried to whack Al'THor since it was obvious by then (to those with enough foresight) that he was LTT reborn. The DO hadn't yet issued the 'no kill Rand' edict so Demandred would have tried.
That leave the CotL as the best target. If you factor in one of the Chosen commenting about events in the south having Demandreds touch then we have our confirmation.
Sooner or later it all connects back to itself.
Because well, it's just that I've been believing it for so long..... (sniffle)
It's so hard to let go. I mean, that guy said Demandred told him one thing, and Taim another, but, well, maybe he disguised himself one time and changed his mind but didn't want anyone to know it and and and...and...
Oh, I don't know. But I just can't let it go. What of all the great theories??? (sniffle)
Aviendha 
Excalibur's Mississippian
Druid's cute, dutiful, if pert, niece
You left out the facial features pointed out by KENCO
(quote)
In almost every scene featuring Taim, we have exactly the same:
"The prisoner's mouth quirked in what might have been the beginning of a smile."
"Taim's mouth quirked again. It really was not a smile; it never touched his eyes."
"Taim's mouth quirked in that near smile."
"Taim flashed that half-smile again."
"Taim turned to Rand, that almost-smile on his lips"
I believe Robert Jordan just wanted to throw us all off, because he felt he'd left too much proof.
but Kenco's go it right
Kidblondeel
Kidblondeel, he's left out a lot more than that! Implying that there are only 3 Taimandred points! how does he dare? I imagine he just didn't want to read the Taimandred theories to see how many points there were.
What about the beginning of book six, where Demandred has a plan "that will require a gambler's luck", but could "hand them everything"? Disguising yourself as Taim and serving Rand without him catching on would require a hell of a lot of luck. But these other theories about joining the whitecloaks. Really. The whitecloaks are nothing. Or Disguising himself as master Norry (chief clerk of Caemlyn). Oh No, he could bring Caemlyn to ruin!! That could hand the Forsaken Everything!!!
What about the absolute lack of fear that Taim demonstrates when he meets Rand for the first time, escorted by hostile Saldean soldiers?
Rand thinks "Only a madman would claim that name if it was not his" and, at his lack of fear "a kind of madness?"
Then consider that Demandred had said, in the prologue, that he had rooted out all fear from himself.
I could go on, but as you can see, there are a lot more than 3 points at my disposal.
Of course so many people still think that Taim is Demandred (I myself thought so until I read Winter's Heart, and part of me still wants to think so...), because there has been so many evidence that he is, and so few that he isn't. Only WH suggests twicely that Taim isn't Demandred, all the other books suggest that he is. And there is no evidence what so ever that Demandred is someone else. I think, Jordan has changed his mind about Demandred and Taim. I'm quite surpriced actually that only two pieces of evidence got so many people to change their minds about Taim and Demandred (myself included) when there are so many other evidence to support the theory.
Kata
I think that we can all agree that Taim (regardless of if he's a forsaken or not) is a servant of the dark. If he is a servant of the Dark born into this era then he's probably more than a DF. He may have been Osangars protege, afterall we know that Aginor/Osangar was diguised as Dashiva. If Osangar was giving tips to Taim then that'd explain why he might know AoL phrases.
By the way Osangar is another mark against the Taimedred theory. We know that none of the Forsaken get along with one another very well, so why would two of them work the Black Tower together. Besides even if Aggy and D-man-dread were tight homies, you'd think they'd brqanch out so as to cause more chaos wouldn't you?
Sooner or later it all connects back to itself.
Well I am a big supporter of the idea that Taim is Demandred, and so far I believe the similarities out weigh the differences.
the facial features speek alot to me, but it might not be that big a deal, Rand is already on the road to being the same way, (remmoved and hardened and real sepressed like)but then again, sorilea and cadsuane are suppose to take care of that.
Robert Jordan seems to use that discription for alot of characters in Wot, like Lan for instance, in the intire series, I think he only laughs once. (TGH, in Fal Dara after the Amyrlyn shows up)
but there is still that they are described similarily in power, in height if I remember correctly, in behavior, in allegiances, and others I've almost definitly forgotten.
If I wrote a series and put in as much planning and foreshadowing as Mr. Jordan, I think I'd read what my fan's thought was going to happen. And if I found out that they already new, then I almost definitly would have tweaked the story a bit to give them pause, and hope that it sustained the mystery a bit.
but that's just me...
Kidblondeel
Oh yeah, i can really see this happening. Demandred is chatting to his protege. Hey, Taimo, hows it going. Say, did i ever tell you that in the Age of Legends Aiel served the Aes Sedai and followed the Way of the Leaf, so it's ironic that now they are warriors, and so i just call them "so-called Aiel", so why don't you too. I just think it's really important that you should know this, and also that you should use my mannerisms.
Yeeeeeah.
Kenco's right, unless of course taim is a forsaken resurrected, and I would be really pissed off if he was, he shouldn't know what he knows unless he's a vessel, there seems to be alot of body switchin goin around, and I don't like it.
Kidblondeel
...Going under the idea that Demandred is using Mazrim as a proxy (we've already learned that he likes proxies) here's a thought. If they should be talking about Al'Thor and Demandred made a comment about those "so-called Aiel' do you think that Mazrim might ask a follow up question on that?
By the way on either this thread or another someone pointed out how suspicious it is that Taim knows old tongue words like M'Hael. Why is that weird? There are still people who learn how to speak a lot of the old words. Maybe Mazrim when he was in Dragon Reborn mode decided to learn as much as he could so that he could make himself seem more like a noble leader of men. If he did I imagine learning a word like 'leader' would be a good idea.
Sooner or later it all connects back to itself.
Has any one figured out how Taim has canaled so long without going mad? I believe that, because of that one question, Taim IS at the very least a high level dark friend, if not one of the forsaken.
That's a possibility, but look at Logain, he lived with it for six years I think it was and never got sick, in fact he didn't show any signs either. But he's not a darkfriend, or a forsaken in disguise, at least I hope not...
Kidblondeel
I think one reason these two pieces of evidence against outweigh all the "for" evidence is because there can be hundreds of things that imply that one thing is true, but it only takes one thing to disprove that. Perhaps Demandred really is Taim, or perhaps everyone is looking at it all wrong. None of the evidence I have seen for it says that it is Demandred without a doubt; in fact, a lot of it could be any number of things. However the two pieces of evidence against Demandred being Taim are stronger. I'm not saying whether Demandred is Taim or not, just that the evidence for is mostly circumstantial while the evidence against isn't very easy to get around. I have yet to see a good explanation for why Demandred told someone (cant remember who) one thing and Taim told him another. Everytime someone brings up Winter's Heart I hear "Well, there was a distance so he didn't recognize Flinn." Okay, fine, but everyone always ignores the other evidence, which was what made me start thinking that maybe Demandred is not Taim after all. I actually didn't notice the Flinn thing til it was pointed out later. So if someone can give me a satisfactory explanation of the above, then maybe I will start believing the Taimandred theory again. But I haven't heard it.
KLitke7967
Maybe he has gone mad!
No, seriously. There was a theory I read a while back that basically said the madness "thins out the barrier between present and past" or something(so when someone goes mad, they're just talking to a past life). Maybe Taim got taken over by his past life, which would explain A. why he hasn't gone mad and B. why he talks like an AoL person.
That's all,
Penguin
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PC hater
Confident, cocky, lazy, dead.
Yes... Very much so...
Healer among the Guardians 
Craziest channeler of them all! 
Trigger 
Taimandred still LIVES... as a proxy
One more evidence