The problem doesn't appear to be a few isolated cases, as two separate threads (here and here) have popped up in the Apple support forums, nor is it a pandemic, as Engadget reports their iMacs are fine. The suggested causes have ranged from a corrupted Snow Leopard build, to a bug that spins down the hard drive, to the nebulous "bad permissions." In the meantime, though, those big beautiful 27-inch screens are playing video like a 128K trying to run Crysis.
A user on the first thread reported that Apple phone support said the problem was with the graphics card, and that it should be fixable in a software update, though another customer who opened a ticket was instructed to exchange the computer itself. Is Apple going to have to smack down another of its chip partners? We shall see.