I am going to agree with all of Imelyen's 4 points. Not only that Snape was actively teaching Harry whilst fighting him, but also that he never attacked. Furthermore, he did not know anyone knew what he'd done on DD's side, so upon learning that Harry knew, why not either kill him or capture him - both he could have done with EASE. If LV wants Harry and Snape wants to please LV, why not bring him Harry?
Also, the whole Unbreakable Vow w/ Narcissa could mean 2 things. One, that it was LV's wish or 2 that it was DD's wish that Snape agree to help. He obviously knew the plan, and if it was option 1, then LV had also told him to help Draco, because he's an evil genius and would have suspected Narcissa to run for help. If it was option 2, then he would have told DD the plan for Draco to kill him as soon as he heard it, thus DD must have told him to maintain his cover at ALL costs. A spy is too insrumental to TOTP's success.
My final clue was: the hand and Defence Against the Dark Arts. Snape said that DD's hand was injured as a result of slow reflexes, but he knew full well that DD had not been in a fight, because he had saved him, and no doubt could guess what had caused the injuries. The fact that he did not reveal that DD was destroying Horcruxes is also proof of his true loyalties. Next, DD is old, trusting, but NOT stupid. He trusts Snape but will not let him near Dark Arts, so therefore, we see his caution. Why throw this away? Was he really THAT desparate for a teacher. I doubt it. This is a part of one large scheme. I mean, he knew all of Draco's actions around school, he may have even known about the Vanishing Cabinet. But he certainly knew all that Snape knew.
Yes, Snape is a slimy weasel, and I know he's dug himself a big hole. Somehow, though, his innocence will be proven in 7. Either by DD's will being read by Harry, or by helping to defeat LV.
Sorry for the long post, but I think I have made a case for Snape, now feel free to argue!
![:D](./images/smilies/icon_biggrin.gif)