I have always believed Demandred to be Taim. There are a few reasons for this, though I do not have the books in front of me to quote from I shall paraphrase as best as possible. In LoC we get our first real glimpse of Dem from his point of view, when he is visiting the Bore. Shortly thereafter we see him discussing events with graendal and (crap i forgot). We never learn what was said to Dem by the 'Great Lord' and I believe this to be intentional. In chapter 6 when first meeting with Rand as Mazrim Taim, Bashere, who has fought Taim numerous times and knows his manurisms and ways better than most, is at first skeptical that this is Taim. Though Taim makes light of that fact saying that he shaved due to the heat, I always got the impression that Bashere recognized the physical Taim, otherwise why all the anger, but did not think that Taim was acting as himself, whether from the way he walked or the way he was talking.

My next point comes at the end of the book LoC I believe, though I'm not possitive on this one, where Taim is seated on his thrown in the BT and states 'as the old saying goes ''let the lord of chaos rule'' This message was only given to Demandred, and if Taim were a new dreadlord, Demandred would certainly keep as much information from him as possible and only pass on orders.

Anyway, first post, I know I could fill out this theory with more speculation and quotes with time, but as I stated I do not have the books in front of me.