To begin with, anyone who wants a well-done analysis of Severus Snape, needs to go read Dave Kopel's essay, "Severus Snape: the Unlikely Hero of Harry Potter Book 7". It was this essay that convinced me to buy the series and read it, and I must confess it shaped my early thoughts.
Reading in so compact a fashion has a certain effect, and I have some definite ideas about what to expect for Book 7. Now, about Snape, one thing I think is obvious about Snape, is that he definitely is not 100% with Voldemoldy. Early in the book, he pretends to Narcissa, that he is privy to the plan involving Draco, yet later Draco's words make it clear that Severus did not know the plan. Also, there is the matter of his confrontation with Potter at the school. Despite his shout to the Death Eaters that Voldemort has commanded that Potter be left to him, there is no reason why they could not have attempted to capture or at least wound Potter - this is a strong indicator that Snape does not want Potter attacked. See also how Snape pulls his punches, not even using spells which could easily and quickly have frozen Potter and put him in immense pain. This is consistent with the whole 6-year history; Snape insulted and demeaned Potter, yet his specific actions were never personally attacking. Consider, for example, the difference between Snape's idea of detention (filling out index cards by hand) and that of Professor Umbridge, who tortured Harry by forcing him to write lines into his own hand, with his own blood, leaving scars visible a year later. And Snape never took advantage of many opportunities to interrogate Potter at length, or physically hurt him at all. I can only surmise that this means there was a force compelling Snape to not only not hurt Potter physically, but even to protect him. It is also obvious to me, that Snape despised this compulsion, whatever it is.
Before going further, it is important to also recognize that Severus Snape truly loathes Potter; it is not at all an act, especially since Snape mocks and belittles Potter at every chance, far more than would be necessary just to play the part of Volemoron's minion.So, it is very unlikely that Snape is totally on one side or the other.
So, with these opposing points in mind, what to make of Severus Snape? I would say, return to the constant elements of Snape, which are the following:
We see these elements over and over again. It's not hard to see how Snape became a Death Eater in the first place. VoldeOopsy would have noted how Snape was an outsider, and played that to his advantage. It also bears noting, that Snape's early talents and interests were to the Dark Arts, plainly in hopes of exacting revenge on all the people who were better-looking and more popular than him, who always had friends and money at hand, and who looked at Snape as an object, not a person. I would go so far as to say MoldyWart trusted Snape a bit more than other Death Eaters, because he saw some of the same emotions in Snape that he had experienced when he was that age, and believed he would know if Snape changed his mind and heart.I take Dumbledore at his word, when he said (repeatedly) that he trusted Snape, and despite appearances, I believe there was a solid reason underneath it all, but Dumbledore never shared it with anyone, because he knew that the secret reason was what kept Snape alive; if Dumbledore told anyone why he knew he could trust Snape, that knowledge would eventually make its way back to the Dark Lord, and that would be the very gruesome end of Severus Snape.I think what happened, was that Snape's relationship with James and Lily Potter changed after school. Snape never came to like Potter, but as time passed his rage faded somewhat, and the effect of Lily on James was to change the man for the better, and soften Snape's mind, not to the point of forgiveness, but to the point of withholding deliberate harm. That was one of Snape's secrets, you see, he knew how to really hurt someone, even kill them, yet he never did so, always holding back his worst weapons so he could tell himself he did the noble thing, he the 'Half-Blood Prince'. But when he told VoMo the bit of the prophecy he heard, thinking to ingratiate himself with the Dark Lord, he unwittingly sent James and Lily Potter to their deaths, and in Snape's mind that created a debt, one which could only be repaid by protecting Harry. Also, Snape came to realize that VultureMortis had no intention of rewarding him for his loyalty, indeed that his half-breed lineage would forever keep him out of the inner circle, and that VoicedeMonkey would never allow anyone to share his power to any degree, that tore it; in Snape's mind VoMo had violated a point of honor, one which changed Snape's course forever.
So, I would suggest that in book 7, Snape will betray Voldemort, picking his time for the best opportunity. It will likely be near the end of the story, but will include protection for Harry, and likely even provide him a weapon of some sort to use against VoMo. Given Harry's mistrust of Snape, he will not realize the gift for some time after it's offered to him, which may likely cost Snape his life. It would be fair turnabout for Snape to give Harry a gift in repayment for letting his parents be killed, and Harry use it in repayment for Snape dying to get it to him.