Apple Thoughts: ATI Radeon Graphics Problem on New iMacs

"In a near-repeat of an issue that plagued the aluminum iMac early on, owners of new iMacs equipped with the ATI Radeon HD 4850 graphics chipset are encountering frequent system freeze issues. Various buyers reporting back to AppleInsider, as well as those in Apple's support forums (one, two), describe an issue where iMacs custom-ordered with the faster video hardware will hard-lock at random, forcing the user to reboot."

Apparently it doesn't even matter if you are running OS X or Windows on the machine, still manages to lock up the iMac. Apple, according to the forums, is aware of the problem and they are supposed to have a fix in with the release of 10.5.7.