Oh goodie!!hehe
Right well most of my theory is from the clues in the books and also from some of J K Rowlings interviews.
First the Facts!! (I cant quote from the books at the mo as I dont have them here)
JKR did an interview just before the 5th book was released and she refused to say whether Harry would even be alive after the 7th book. (she has consistantly refused to answer this!!)
At the Edinburgh Book Festival (
http://www.mugglenet.com/jkrebf.shtml ) she gave another interview and said that the 2 questions the readers should be asking are, not why did Harry survive but why did voldemort live, and why didnt Dumbledore kill Voldemort in the 5th book?
(Im sure theres more but I just cant think at the moment, will edit if anything comes to mind!:P)
Well my theory is that when Voldemort attempted to kill Harry we know that because the spell was so powerful it left part of Voldemort in Harry. I think that it also left a part of Harry in Voldemort and that the reason that Voldemort didnt die was quite simply because Harry didnt die.
I think that Harry will die in the last book because it will be the only way to completely kill Voldemort and I think that Dumbledore is the only one who knows this so far and that is why he is so protective of Harry and also why he didnt try to kill Voldemort. Quite possibly I can see Harry sacrificing himself to protect the rest of the world.... (Dragonheart anyone??)
Of course this is just complete theory and I dont know how the other characters would fit in and Im sure that if Im even semi right that JKR would twist it in far more ways than I can imagine. But it does kinda make sense to me at the moment and it certainly would give the Harry Potter saga complete closure.