I found him after reading THBP - I just couldn't believe that Snape was a bad guy! But now I've got to go through and read the books again because I read them as children's books and totally skipped over all the clues.
I like the "Snape's in it for himself" theory -- which probably makes him the most dangerous character of the entire book.
Looking back, we see that Snape has been threading a course between good and evil throughout the series, and even before the series began.
We see in Snape a direct comparison to Pettigrew: they play the same role, but on different sides. Pettigrew pretended loyalty to Dumbledore; Snape pretends loyalty to Voldemort; and both come down on the side that best serves their interests.
Pettigrew was eventually forced to made his choice, which he did, and which gave us the entire basis for the HP series.
Snape's test comes at the end. His loyalties will be strained to the limit in book 7 -- between the forces of Good (embodied in Harry, who has his mother's eyes), and Evil (embodied in Voldemort). Snape has now reached the same point of having to make a decision. His struggle to decide will undoubtedly drive the ending of Book 7.
That being the case, it puts the prophecy in a completely different light. "The other" in the prophecy refers not to Harry or Voldemort, but rather to Snape.... Either Harry or Voldemort must die at the hands of The Other (Snape_, and Neither of them can live while The Other (Snape) survives.
Thus we see the outlines of the ending. Snape will die in the process of killing either Harry or Voldemort. Since JKR has hinted that a look at her faith will give away the ending, we know that a Voldemort victory cannot occur.
Thus, it's clear to me that Snape will kill Voldemort and die in the process. He'll be redeemed, Harry's soul will remain untorn by killing Voldemort, and Good triumphs.
IMHO....