I think, since Snape has always been a "spy" of sorts, that in making the binding pact he made with Bellatrix to help Draco... I think he went afterwards to Dumbledore and told him everything and Dumbledore found a way to help him : complete his promise, NOT blow his cover, and stay alive.
So in short, I think Dumbledore's death has been faked and I do not think that Snape has turned against him. Perhaps Snape's cover was about to be blown or some such thing.
I think we'll be seeing some major plot twists in this last book.
On a side note, I believe Harry himself is a holocrox (spelling?) in that he must somehow purge himself of the powers that Voldemort shares with him in order to kill Voldemort. It basically states that in order to really kill Voldemort you must destroy all the holocroxes, so either Harry must sacrifice himself in the end, or somehow lose part of his power.
Those are my random thoughts..
Glad to see there are Harry Potter fans here too!