Apple has apparently admitted that the discoloration on the wrist rests of some MacBooks (mostly white ones) is actually a manufacturing defect as opposed to a "cosmetic" problem caused by improper handling by the owner (whatever that means).
Cosmetic problems that don't affect the functionality of the device aren't covered by AppleCare. Softpedia has the details:
A small, but still significant number of people have been reporting an unnaturally fast discoloration of their new Apple portables, especially in the areas where the hands come into contact with the device. While initially passed off as improper handling by their owners, it turns out that there is indeed something strange about the plastics used in some models of the white MacBooks, that causes the discoloration, sometimes after not even two weeks of use.
If you have the problem you should contact AppleCare and let them know and they will arrange to replace the top case with a new one that uses a different plastic formulation. It appears that the original MacBook plastics were too porous which led to staining after oils become embedded in it from prolonged use. Apple changed the formulation of plastics in the production line already, so the newest models arriving in retail stores shouldn't have the same problem. The newer top case is reportedly much smoother to the touch than the previous MacBooks which feel slightly rougher. Did your MacBook stain?