Look, when he died, (sorry for the spoiler!) THERE'S NO EVIDENCE!
I mean, it says that he falls of the tower, but how do we know he's dead? It was never clearly stated, except for the quote: "Snape Killed Dumbledore", by Harry. But it was never clearly stated!
Theres already a thread open on this located here: http://www.frihost.com/forums/viewtopic.php?t=9384&highlight=
Yep. There is already a similar thread about it.
Even though it's been many months, some people may not have read this book yet. Please put a spoiler notice in the title to make sure you don't ruin it for anyone else.
Sorry but I tend to be cruel like that by putting spoilers to each and everything that crosses my mind ...