1. Nihilus is not Revan. It was completely plausible that that would turn out to be the case and of course one of a few endings that were anticipated. It could well have been the case that when we got to the Bridge he reveals this, but he doesn't, he just dies, and we don't hear any more of him for the rest of the game. If he was Revan then this would have been revealed... Of course... Because otherwise what was the point in it being so, and why have they not told us such an important thing as the fact that we just slayed the mighty Revan? Also, we're told exactly where Revan is at the end, and Kreia makes it very clear that he is still alive, fighting the Real Sith in his own way. The entire KOTOR I and II plot centres around Revan and the things he has done, and so for us to kill a minor character and then one day to find out, by the way, that he was Revan... you didn't get to talk to him, or find out what he's been doing, or what he found in Unknown Space, or answer ANY of the many questions which have been formed... would be apauling and is NOT what has happened. Nihilus DIED. If there is a III nobody will care enough about Nihilus for the late revelation that that was Revan to mean anything. Except that there is no story path for this third installment to follow. Believing him to be Revan before you play the game is fine, but believing it after you've completed the game is not very intelligent.
2. It is clearly stated that Nihilus is born out of the devestation at Malachor V... Therefore the cut-scene with Kreia, Sion and Nihilus has to be between the times of KOTOR I and KOTOR II (or during KOTOR 1).
3. Nihilus being the Exile... This is another theory which, like the Nihilus/Revan one, is entirely plausible for most of the game, and there ARE a lot of similarities between them. However, as is the case with Revan... IF THIS WERE TRUE WE WOULD BE TOLD. How could they possibly just not tell us something as important as this? Nihilus is a minor character who had 2 or 3 cut scenes, 2 or 3 lines, and will definitely not be returning in KOTOR III... So there is no possibility of it being revealed in III... Therefore, it is not the case.
Nihilus looks cool, and is on the box, but is nowhere near important enough to be Revan or the Exile.