Apple has released MacBook SMC Firmware Update 1.1, which aims to improve the MacBook's internal monitoring system and address issues with unexpected shutdowns.
A number of MacBook owners have had their MacBooks intermittently randomly shut down on them, an issue which has come to be known as MacBook RSS (Random Shutdown Syndrome). In a recent summary of the issue, including a pending class-action lawsuit, AppleInsider quotes Ogrady's PowerPage that nails down the root cause of the issue to the following:
"Essentially the heatsink can expand during use, and comes into contact with the lead from the [thermometer's] sensor cable," reads a more detailed explanation posted at Ogrady's Power Page. "A short circuit results, and the SMC (System Management Controller) pulls the plug. Once the system cools down, the heatsink [recedes] and the contact is broken."
As is the case with all firmware upgrades, Apple cautions users to not interrupt the update. Detailed information regarding the update can be found here.