Too soon to tell how big a problem this is, but it looks like a pain for the people experiencing it. Apparently, some MacBooks are suddenly, randomly shutting down. Just turning off completely. Details here. I’ve got a call in to Apple for comment, so check back for updates.
UPDATE: An Apple spokeswoman directed me to this page on the site that tells people having the problem to contact AppleCare. Not much help. I’m asking for more details about the problem.
Hardmac.com has this: Some are saying Apple has indicated it’s an incompatibility problem between some motherboards and some cooling systems. “The previous cooling system was expanding more than expected when draining the heat from the CPU, and as a consequence was creating a short-circuit of one of the temperature sensors leading to the emergency shutdown of the notebook.”