If your MacBook has been shutting down since the last update, get over to the Apple Support site and download the new SMC Firmware update.
The SMC Update improves the MacBook’s internal monitoring system and addresses issues with unexpected shutdowns. This update is recommended for all MacBook systems, including those that received warranty repair.
My MacBook has been shutting down on itself around 5-6 times a day, I just went and downloaded the Firmware update. I’m hoping that it will FINALLY solve the biggest problem I’ve been having with my MacBook (I’ve already sent it in once to have the motherboard and optical drive replaced). If the firmware update does not fix your MacBook, you could be facing a Logic board problem, and you’ll have to contact AppleCare.