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.
While Apple typically does not support cosmetic issues that does not affect the operation of the computer, users that have the issue can contact AppleCare to arrange for a replacement of their top casing.
Update: Macworld.co.uk claims that the above report is incorrect, and says that according to Apple representatives that they have talked to, the staining is still under investigation by Apple, and Apple is not officially recognizing the issue as of yet. Still, Apple is encouraging users with the issue to contact AppleCare.