Apple has released an update for iMacs equipped with ATI Radeon graphics, particularly the HD 4850 chipset, which seems to have been the cause of intermittent system freeze issues on the respective systems. Additionally, the new firmware should fix wake-from-sleep problems in Boot Camp on the very same iMac models.

From Apple's Support section, the new firmware's description goes:

About iMac EFI Firmware Update 1.4

This update fixes intermittent system freeze issues for iMac computers with ATI Radeon HD 4850 graphics and fixes wake-from-sleep issues in Boot Camp.

To complete the firmware update process, please follow the instructions in the updater application (/Applications/Utilities/iMac EFI Firmware Update.app). The updater will launch automatically when the Installer closes.

After the firmware is successfully applied to your Mac, your Boot ROM Version will be:
IM91.008D.B08

Weighing in at 1.7MB, the iMac EFI Firmware Update 1.4 requires Mac OS X 10.5.6 or later and supports several languages, including Deutsch, English, Français, Chinese, Español, Italiano, Nederlands, Dansk, Norsk Bokmål, Polski, Português, Português Brasileiro, Pусский, Suomi, Svensk, Japanese, etc.

iMac users can check to see whether or not they need the new update by using System Profiler. Choose About This Mac from the Apple menu, then More Info..., then navigate to Graphics/Displays and check with the Chipset Model. If the Chipset Model listing does not indicate that you are packing an ATI Radeon HD 4850, then you don't need this update. You can also confirm the version of the Boot ROM you have installed on your computer by using the System Profiler.

The latest iMac EFI Firmware Update (1.3), released in April last year, fixed several issues to improve the stability of iMac computers. Apple's all-in-one desktop computers remain the most stable, with just one firmware update released in over a year.

Download iMac EFI Firmware Update 1.4 (Free)