Apple today released iMac Graphic FW Update 3.0, addressing an issue that could cause hanging and freezing on 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 download weighs in at 481 KB and requires OS X Lion. Apple does not specify on the download page which iMac models require the update, so iMac users may want to run Software Update to see if it is offered for their specific machines.