I don't know if i'm replying to the right people..but I sure hope that somehow Snape and Dumbledore had this planned and Snape really is still working for the good side..otherwise, I'll be really dissappointed in the books, which I've loved to this point..I couldn't believe the reason given for Dumbledore trusting Snape was that Snape said he was SORRY? Huh? Surely he's going on more than that..I like the idea that Dumbledore was planning his death much better..
So, if they did make an Unbreakable Vow or something similar way back when Snape returned repentant, who sealed their vow? My money, if that is the scenario, is on Lupin.