I think Snape is doing Dumbledore's bidding, going all the way back to when he approached DD, remorseful, at having set LV on the path to the Potters. I think the reason why DD has trusted Snape over the years is that the two of them made an unbreakable vow, back then, that Snape would always do everything in his power to protect HP's life, since he was responsible (partly, at least) for the deaths of James and Lily. And Snape has always done that, starting in Sorcerer's Stone and going right through HBP. At the end of HBP, when the Death Eaters are all running away, Snape not only
didn't kill HP, as he could have, but he also didn't
abduct him, which would have been just as easy and--for a true follower of LV--abducting HP at that point would have been the logical and most rewarding course of action. He couldn't do it, though--it would have undoubtedly led to HP's death.
So right now, everyone is convinced Snape is the bad guy (excpet for maybe Bellatrix and LV). If it turns out he's really one of the good guys, how will anyone else find out? A neat bit of continuity would be if DD had left HP his memory of Snape having made an unbreakable vow to protect him, if that is in fact what happened. It may even bring HP back to Hogwarts for a day or so. There don't seem to be a lot of pensieves sitting around. All conjecture, obviously, but what fun! At least it gets my mind off of our real estate sitch.
All true; plus, I read somewhere, Snape took the time to give Harry one last lesson -- in what to expect when he fights Lord Voldemort -- before he fled.
I can't remember anticipating a book more than I do Book Seven.
...hard as that is to admit!