Apple has released an update for the firmware of the Radeon HD 4670 and 4850 chipsets that are present in the new 27-inch iMac systems. A number of people have reported problems with the displays flickering and images appearing distorted and corrupted. This update addresses these issues.
The update is a small 683KB application that should be available in Software Update for applicable systems, and can also be downloaded from here: http://support.apple.com/kb/DL984.
Apple recommends you apply the firmware update with as minimal interference as possible, so be sure to quit all open applications and unplug any external devices before running the installer. You may consider booting into safe mode before running the update, just to be sure all third-party add-ons are disabled.
Your graphics ROM should be either "113-B9110C-425" or "113-B8030F-260" after updating, which can be checked in the "Graphics/Displays" section of the System Profiler application.