Reading online at The Floo Market, I found a timeline of Snape's life...with interesting results.
The interesting bit I noticed was this:
1981(before October): (Snape) Approaches Dumbledore and offers to be secret agent for The Order of the Phoenix. Snape also applies for the post of Defense Against the Dark Arts teacher at Hogwarts; Dumbledore hires him as Potions Master instead (OP17).
So if this bit of information is true, that means that Severus Snape turned to Dumbledore BEFORE Lily and James Potter were murdered by Voldemort.
Which also means that it is likely that Snape was the one who tipped off Dumbledore to the fact that Voldemort was going to attack the Potters.
What do you think?
I'm also thinking back about Snape's Unbreakable Vow with Narcissa Malfoy. Shouldn't the vow have been served when Snape murdered Dumbledore? If that's true, why did Snape help Draco escape? He could have let the kid fend for himself, but he didn't.