Customers participating in Apple's official support discussions (one, two) have reported that the new iMac will technically continue to run when the freeze occurs, but that it becomes impossible to control or switch to programs without a hard reset.
"Whatever was going on at the time continues, so if you were listening to music it continues to play," one user wrote. "The mouse pointer moves but you cannot select anything. Tabbing through apps doesn't work."
Significantly, some can still control basic functions from the keyboard, such as pausing music or activating the Caps Lock key.
The issue appears to be related to the ATI Radeon HD graphics hardware and its drivers, but has proven unpredictable. While some customers have said they can reliably trigger the freeze by running games, iTunes, or other programs that push the video chipset, others have had the lockups occur at seemingly random intervals or after running the iMac for a long time.
Those who have contacted Apple for help have found that the company is aware of the issue, but hasn't yet responded with a permanent fix. Both the 1.0 and 1.1 iMac Software Update packages released since the iMac's August launch have included "important bug fixes" which users later discovered were almost exclusively centered around video drivers, but none of these included a fix for the freezing issue.
Some users' systems even appear to have been made worse by the latest of the updates. AppleInsider's own review unit, while stable during the review period and for weeks afterwards, now suffers from the same issue since the 1.1 update and will occasionally refuse to respond regardless of what was running at the time.
A handful of users claim to have repaired the issue by manually reinstalling earlier drivers, though this unofficial fix has not been successful for AppleInsider as well as other users. However close Apple may be to a fix, the widespread nature of the problem is making some prospective buyers think twice before purchasing the system so soon into its release cycle.
"I would hate to get a new iMac, and then experience problems with it right out of the box," said one visitor to Apple's discussions.