In recent months, some users were experiencing freezing problems with their iMacs, apparently due to faulty drivers used for the included ATI Radeon HD graphics cards. Apple aimed to fix the issue by the end of October and has just now released the "recommended" update for 20" and 24" iMacs with 2GHz, 2.4GHz and 2.8GHz processors. Apple has two separate updates available for download: iMac Software Update 1.2 for Tiger and iMac Software Update 1.3 for Leopard. Although the company originally stayed silent on the issue, it is great to see that once they acknowledged it they worked quickly to release a fix. Some users are reporting, however, that the issue remains unfixed.
News source: DailyTech