Apple now acknowledges the Random Shutdown Problem in MacBooks

After recognizing the discoloration issue of White MacBooks, Apple now acknowleges the Random Shutting Down of a number of MacBook units and is recommending those who are affected to contact AppleCare as soon as possible.

MacBook: Shuts down intermittently

If your MacBook is shutting down intermittently, please contact AppleCare for service.

source: http://docs.info.apple.com/article.html?artnum=304308