While some people may continue to believe that Snape is a traitor and an evil man, I still have my beliefs that he is working for the Order in some way. I was searching online recently and found some more evidence of this, straight from JKR's mouth.
There's an important kind of redemptive pattern to Snape.JKR: He, um, there's so much I wish I could say to you, and I can't because it would ruin. I promise you, whoever asked that question, can I just say to you that I'm slightly stunned that you've said that and you'll find out why I'm so stunned if you read Book 7. That's all I'm going to say.
This is from a chat in 1999, before book 4 was released. So I think JKR obviously had this all planned out and we will find out that Snape's true loyalties lie with the Order. His "redemptive pattern" will be continued when he redeems himself with the good side. Everything about Dumbledore's death will be explained thoroughly.
I was even thinking about how Snape could convince the Order of his true loyalty and thought that maybe he could show them one of his memories via the pensieve. Dumbledore and Snape could be there and their whole plan could be easily explained. Dumbledore would be seen planning his own death and perhaps even giving the Order members a message. I don't know if this could work out, but I think it could be a possible way for the Order to believe Snape. The pensieve never lies and always shows the viewer the truth, not the perspective of the memory holder.
I just know that Snape could never fully be evil. I know that his loyalties always will lie with the good side.
**The chat transcript can be found at this address
http://www.quick-quote-quill.org/articles/1999/1099-connectiontransc.html **