Apple - Support - MacBook Bottom Case Replacement Program

Apple has determined that under certain circumstances the rubber surface on some MacBooks may separate from the bottom case of the system. MacBooks shipped between October 2009 and April 2011 may experience this issue.

Apple will replace the bottom case of any affected MacBook, free of charge, that exhibits the issue.
Owners of White MacBooks with peeling rubber bases are now eligible for free repairs of the bottom case, in or out of warranty. I don't believe the standard AppleCare warranty applies to the bottom case, so this applies to all users regardless of your warranty coverage status.

See the provided link for more details.