I just got a surprising free replacement while looking to check a 2006-old battery life at Techroom (Authorized, great service). It took one day to order the parts and 10 minutes to finish them.
Your Macbook is covered even you're not in Apple Care or any kind of warranty.
Apple is privately acknowledging an issue with the enclosure on some of its white 13-inch MacBooks, which in some cases have seen the formation of hairline cracks during normal usage patterns.
This was back to April, 2009 and I am a happy exciting kido like getting a whole new macbook for Christmas gift as early as a back-to-school season.
Check Apple Insider to see the private announcement.