Which, like I said in my previous post, Snape will become the new Headmaster of Hogwarts!! It won't be Minerva, she's getting on in years. There's no other professor that can handle the position. Snape was trusted completely by Dumbledore, and I bet he left something behind for Snape to carry out his last wish and take over as Headmaster!
After the way book 6 ended? I find that highly unlikely. I thought that Snape took off after he killed Dumbeldore. I think that it's likely that if he's playing the double agent, then he'll see things through that he might get close to Voldemort for book 7. And I can see Professor McGonagall taking over Hogwart's for that book.
The one thing that keeps coming back to me is Sirius Black. Somehow I don't think he died in the Department of Mysteries. I think he's going to be back in book 7.
Don't forget wizards have longer life spans than muggles. McGonagall is supposed to be roughly half Dumbledore's age (75 and 150) and he was still pretty spry up until the end.
I'm sure Snape will turn out to be good at the end, but I hope it is more of a he is out for himself thing. He will help Harry defeat Voldemort because Snape wants to be the next Dark Lord. I think it would make more sense than Snape killing the most powerful wizard for the good side just to keep his cover with Voldemort.