The 27-inch iMac hasn't been out long, and users are already reporting performance problems with the new model.

There are two Apple Discussion Board threads [one, two] of users reporting problems with laggy Flash-video playback, a bug that causes the hard drive to spin down, corrupted build of Snow Leopard, and possible bad permissions problems. 

Apple has yet to validate any of these claims, but if you own a 27-inch iMac and are you're experiencing with any of these issues, it might be worth a call to AppleCare.