If anything, I would say that if it were true it would probably be in the Half-Blood Prince, since Severus made the Unbreakable Vow to protect and help Draco with his mission of killing Dumbledore. That would probably be the best place to look. :)

I think I read it somewhere too, can't really remember either, and it is often mentioned in fan fiction, so i'm not entirely sure if it's true but I like to think he was or at least I always thought he was.

I can't remember reading it in the books that SNAPE was Draco's Godfather.I think this idea has come about with Sirius, who was James Potter's friend, being Harry's Godfather.Although he was friendly with Lucius Malfoy,Snape wasn't in the same year at Hogwarts because he was younger and they weren't best friends.People have made assumptions here, I believe.He just got the job of protecting other people's kids!