Lily, Severus, and the Killing Curse
I commented in a review, and I thought I'd basically cross-post here.
The most likely explanation for Harry Potter surviving the Killing Curse is that Lily *sacrificed* her life for his. In other words, Voldemort kept his word to Severus Snape, and tried to keep her alive - but in trying to kill Harry without first disposing of his mother, it allowed Lily to (probably in a burst of accidental magic) superimpose her life between the curse and Harry. The curse, running into an unexpected barrier, rebounded and killed Voldemort. But the cost of the killing curse is a life, so Lily died.
That would explain Harry Potter's memories in the books (green light + pleading/etc), and it would go a long way to explaining why Snape hates Harry - because Snape probably knows enough Dark Arts at that point in his life (as well as no doubt researching it) to realize what happened. Harry, in other words, is responsible for Lily's death; if he had died, Lily would have lived. It also explains why Snape was willing to help Harry, even after Lily died (and Dumbledore essentially broke his promise of protecting her, unlike Voldemort who kept it to his detriment): because Lily was willing to die for Harry, so to let Harry die after that sacrifice would be tantamount to making her death worthless.
Otherwise, the whole rebounding thing would be a lot more common.
I'm sure someone other people have used the same explanation. But it's worth repeating.