I know it sounds crazy, but he might not leave early after the speeding ticket and down year he had. Both probably knocked him out of the top 5 if not the top 10 altogether. Plus he is going to need to get surgery, so he won't be able to do anything at the combine. Factor that in with how close we came to BCS glory this year, and how much talent we are already returning, he might decide staying is in his best interests. I am sure he'd love to be the first number 1 overall pick we've had from USC on defense, and he won't be able to accomplish that goal without returning.
It's also possible he's just lazy and figured he's first round no matter what, so screw it. The deadline for him to officially declare is January 15th, so keep your eyes out for an announcement one way or the others.