I've been considering what the cover could REALLY mean...
I think it would be a really cool twist to have Snape be the actual villain, but he doesn't pack as powerful a bunch as we've seen Voldemort. Not only that, but the entire series has put stock in taking down Voldemort... Horcruxes, waging a war against the black arts, etc. If Snape were to all of a sudden become the main baddy, I gotta tell you - I would have mixed feelings about it.
What I am thinking, since he and Harry have a connection, that there is more to Voldemort than what we've seen of him... There is something huge still missing from his story, I think.