The bottom of the 2009-2011 model white MacBooks has a rubber panel attached to it. In some cases people have found that the rubber panel would begin to separate from the bottom of the MacBook due to an issue with the glue used.
Apple has posted a support document stating that they have acknowledged the issue and will now replace the bottom part of any white Macbook that exhibits the problem. The replacement includes a new bottom case, screws, a philips head screwdriver and instructions, or you can visit an Apple Retail Store with an appointment to have them replace it.
The program does not extend the standard warranty coverage and applies to any affected MacBooks for two years from the purchase date. This means that you should be able to get the bottom panel replaced even if you’re out of warranty by as much as a year.