I don't think Snape can ever be "good" again. No matter what DD believed.
I think this issue will be one of the big debates -- perhaps the biggest -- between now and the publication of the final book.
And I'm of the opinion that Snape will be instrumental in Voldemort's ultimate downfall according to a Dumbledore's plan. Snape may never be "nice", but I think he's in the anti-Voldy camp because:
1. Dumbledore thinks so, and he's seldom (if ever) wrong about character.
2. It's dramatically right. We think for volumes that Snape is nasty, suspect he's good -- and then find out (we think) that he's the ultimate traitor. The dramatic reversal that shows us he isn't will be pretty powerful stuff.
3. He and Dumbledore cooked something up -- something that upset Snape and led to a heated discussion with DD. Just as DD had to twist Harry's arm to follow his instructions to the letter about the liquid covering the locket, I think DD had to exert his will to get Snape to do as he ordered -- including killing (or seeming to kill) him. As I noted in another thread, the emphasis on non-verbal spell-casting in this book was (I think) a clue that, although Snape SPOKE the Avedra Kedavra curse, he might have been THINKING something else. And DD's pleading look -- which might have seemed like he was begging for his life -- might easily have been DD pleading with Snape to follow through on his promise and further DD's plan.
4. Snape could have done Harry considerable harm in the final chase but actively refrained from doing so.