The Macbooks between 2009 and 2011 have rubberized bottoms, and while these are actually pretty cool, they have a tendency to detach from the hard plastic underneath. Apple addressed this problem with a repair program in 2011, which offered a free repair two years from the date of purchase.
Today though, Apple has announced it is extending that policy to four years, and there are a couple of different ways to get it repaired if you are affected (via MacG).
From the support page:
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.
There are two options to get it repaired free of charge. You can take it in to an Apple Store/Authorized Service Provider or you can do it yourself with the kit that Apple will be glad to send you. If you feel in the mood for some DIY, you can order the free kit here.