MacBooks shipped between October 2009 and April 2011 may exhibit a problem where the rubber surface may separate from the bottom case.

Apple has announced a MacBook Bottom Case Replacement Program to address this issue, where Apple will replace the bottom case of any affected MacBook free of charge.

MacBook Bottom Cases can be replaced at any Apple store, by Apple Authorized Service Providers, or you can even order a self-service replacement kit.  http://www.apple.com/support/macbook-bottomcase/