If you have an Intel MacBook that is spontaneously shutting down, for no apparent reason, slide on over to the Apple site via this link.
Answer a few questions and, if your MacBook meets the requirements, Apple will repair it FREE of charge.
Most MacBooks are under warranty anyway but Apple has acknowledged a problem with certain machines. Problem appears to be with the logic board and heatsink.
http://docs.info.apple.com/article.html?artnum=304308
P

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic