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.
The freezes occur regardless of what software happens to be running and aren't even connected to whether or not the user is running Mac OS X; some testing the issue note that it happens when running Windows in Boot Camp, where Mac-only drivers wouldn't be a factor. None of the typical troubleshooting steps have an effect on the flaw, including resets for the PRAM or the SMC unit that are often used to diagnose hardware-only problems.
What, exactly, is rendering these systems unusable remains a mystery, though the symptoms will be eerily familiar for early adoptees of the aluminum all-in-one desktop. In the fall of 2007, owners (and AppleInsider) reported an interface freeze issue where the interface would become virtually unresponsive, even while some tasks would continue to run. The new issue is more severe as it appears to halt all activity, though in the previous situation Apple ultimately still needed to fix the freezing through a firmware update that modified the behavior of the video chipset itself.
Apple may already be aware of the issue. Some of those calling the company's support line have heard from technicians that there are numerous reports and that a fix is in the works, possibly arriving along with the looming Mac OS X 10.5.7 update. Some are being offered replacement systems after unsuccessful troubleshooting.