Well ... looky at what came out today ...
As reported by MacRumors:
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.
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.
Two notes ... this proves yet again that Jason O'Grady makes up whatever news he can to generate hits without regard to the truth, thorough investigation, or promotion of "hope this isn't true".
Also, I have read in several places that the new matte finish on the MacBooks gets dirtier easy ... it's not staining as much as it is oil from the skin getting deep down into the plastic, getting hot, and appearing to be discolored. From what I have read, it's easy to remove with scratch remover ... and has the side benefit from protecting the MacBook from scratches and future staining. I do think Apple should look into this, but it's more an issue of dirty user's hands than faulty Apple product manufacture.