Apple released a new iMac firmware update yesterday, iMac Graphic FW Update 3.0, which is intended to fix issues with hanging and freezing on certain iMac models.
About iMac Graphic FW Update 3.0
This firmware update fixes a graphics issue that may cause an iMac to hang under certain conditions.
The update weights just 481KB and requires OS X Lion. Also, Apple does not specifically state which iMac models are eligible for the update, so it may be a good idea for iMac users to run Software Update on their machines to see if the update is available.