The lower case. Apple's using wide, thin black rubber feet on the Unibody Pros, a departure from the small stubby bumpers on the Aluminum revisions.

There are ten Phillips screws around the perimeter of the lower case to remove:

Seven 3 mm screws

Three 14 mm screws

The computer's serial number is engraved in the aluminum lower case, not the upper case like the other Unibody machines.