The issue is stemming from the 15 and 17-inch models, each of which feature Sandy Bridge Core i7 quad-core CPUs and AMD Radeo HD graphics processors.
A thread entitled "MacBook Pro 2011 17" hard freeze" on the Apple discussion forum is nearing 70 pages, with most users complaining about the crashes whenever using multiple apps, especially video intensive ones like iMovie or gaming.
One user, dT explains, was able to pinpoint the problem:
The issue occurred when running CPU-heavy processes along with programs that caused graphics processing to shift from the new Sandy Bridge processor to the descrete AMD Radeon graphics processor.
The OS X 10.6.7 update included fixes for "graphics stability" but some users are still having issues.