I don't really consider it a spoiler, since it's just the name of his character and hasn't been officially confirmed by J.J. Abrams or any of the others, but for the sake of anyone who doesn't want to know, I'll put it behind a jump.
According to TrekMovie, the Sherlock actor will be playing Khan. If it's true, I'm not enthused. Using Khan as the villain in the second movie just seems cheap and lazy, like they couldn't be bothered to come up with a new villain. Don't get me wrong, I want to see Khan again, just not in Star Trek II. Third or fourth movie, sure, but not the second.
TrekMovie no longer considers it a rumor, but until it's officially confirmed, I am.