Actually, I now think Snape's Patronus is Lily Potter or some unmistakeable symbol of her, similar to Harry's Patronus resembling James. That would certainly explain why JKR has been so cagey about revealing what Snape's Patronus is, even admitting outright that she can't give the answer because "it would give too much away". If Snape's Patronus were merely a griffin, there'd be no reason for her to be so cautious.

The other alternative is that Snape's Patronus is something that by its very nature proves that he is unequivocally good, but I have a hard time imagining what that could be.