Apple acknowledges problems with Aluminum iMacs; promise solution
Users participating in the Apple support forums (at least in two threads 1 and 2 ) have reported that the interface of the new Aluminum iMac freezes without apparent cause, leaving the computer locked (but working normally in background) to perform any action until it is restarted. The issue appears to be related to the ATI Raedon HD graphics card and its drivers, but reproducing the error to find a specific cause is hit or miss. Apple has acknowledged the problem and has indicated that it is working on the matter, although it still does not know what the causes of this problem are.
Apple is currently working on this matter, which according to the Cupertino company “affects a small number of iMacs” and hopes to release a software update that fixes the problem as soon as possible.
Some users have managed to regain control of the computer by launching a game or other program that generates a work request to the graphics card. The problem seems to occur at random intervals or when the computer has been on for a long time.
Those who have contacted Apple have received a reply that they are aware of the problem. The two updates released for the iMac that fixed major issues focused exclusively on the graphics card, but neither fixes this issue, and in fact, the latest update seems to cause this issue more often. Some users seem to have managed to avoid this behavior by reinstalling drivers from previous versions.