
Jason O'Grady posts on his ZDNet blog that Apple has acknowledged and identified the cause of a staining issue that was affecting a small but significant number of MacBook owners.
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.