I bought my MacBook in December 2010, the bottom case on it has failed twice. Sure enough, Apple has opened up a replacement program for the bottom case.
Apple doesn’t go into great detail as to what may fail, but basically a couple of things can happen. One, the thin metal can warp if you open and close the bottom case many times (especially if you upgrade hard drives a couple of times, add in an SSD to replace your optical drive, and upgrade the RAM… like me). The other is that the rubber on the bottom can delaminate, causing the MacBook to look like it has skin cancer.
The solution is simple (now, at least), just head to Apple’s repair extension program page for the Late 2009 MacBook.