Ars Technica:

Numerous owners of the latest 15" and 17" MacBook Pro models have taken to Apple's support forums to report an issue that causes their systems to freeze, with one user going so far as to successfully reproduce the problem on every floor model at a local Apple Store. Apple hasn't officially acknowledged the issue, but the company is allegedly working on the problem, according to another customer. Although Apple has fingered drivers as the culprit, the problem appears to be related to running GPU-intensive tasks that lead to overheating.

This has happened to my brand-new, 15-inch, quad-core MacBook Pro at least twice—I'll come back and wake it from sleep after a couple of hours, and the screen will come on and the mouse cursor will work, but the clock stays frozen at the moment the crash occurred and generally nothing works.