...or rather, the probability, of Snape secretly being in love with Lily Potter while they were going to school at Hogwarts. He brought it up to me, and I remembered the passage from GoF that mentioned how Lily was always kind to Snape during their school days. If he was in love with her, it would explain his his horror and sudden change of heart when he realized the information he gave Voldemort directly lead to James and Lily's death. He was, in effect, responsible for killing the woman he loved.
Which would also explain Snape's occasional and inexplicable bouts of reluctant fairness and covering for Harry who, as he has repeatedly been told, "has his mother's eyes." Harry, however, does still take after James, who Snape hated. How much must it suck to see a child who reminds you every day of the woman who you loved and lost, especially when you lost her to the man you hated most.
This ties in with Rowling's theme of 'unconquerable love' and love being the secret that Voldemort could never understand. Snape has been trying to attone for his actions ever since Lily's death, by working at Hogwarts and watching over her son. He may have a difficult time ahead convincing everyone that he was only acting on Dumbledore's orders, and I think that, ultimately, he's going to have to sacrifice himself to prove his honesty and goodness to Harry, because at this point Harry is way too jaded against Snape. I did find it interesting, as well, that Snape refused to allow Harry to get out an Unforgiveable Curse when Harry attacked him, pointing once again to the fact that Snape really does have Harry's best interests at heart...albeit begrudingly. Heh.
Credit goes to Chenpion, because he'll want it, for brining my attention to this theory. =p It does make sense, though. I don't know if anyone else has discussed this yet, because I only read the book yesterday and I didn't want to read the spoilers going up, but it's interesting, and it helps me not to loathe Snape so much.