Its bad apple quality, happens to lots if not most plastic macbook owners in they're lifetime. On the plus side, if you're in warranty they'll replace it for free.
I hope it doesn't happen to me, mines a 4th revision model so i might be ok.
So far, everyone I know with one pre 2008 has had the problem, everyone with a early 2008 model is fine thus far.
When did you get yours?