Robert Jordan had openly said that Taim is not Demandred. That said, I can see many reasons why people would still think otherwise.
I have always questioned this because if you look at his response to the question he actually doesn't say that at all. The question was:
Week 4 Question: At recent book signing following the release of Crossroads of Twilight, it was reported that you confirmed that the Forsaken Demandred has never posed as the man known as Mazrim Taim, who was introduced to Rand at the beginning of Lord of Chaos. Have you confirmed that Demandred has never posed as the man known as Mazrim Taim, leader of the Black Tower?
Robert Jordan Answers: Yes. Demandred has never posed as Mazrim Taim. All right, those of who fell over from the shock of a simple, straightforward answer can get up off the floor now. Sometimes, simple and straightforward can be the most devious of all, as any student of Aes Sedai will tell you. <Maniacal laughter from the shadows!>
http://www.wotmania.com/faqtopic.asp?ID=152Posed is the key word. Demandred doesn't have to pose as Taim if he really is him. just my 2 cents