Apple has just releases the firmware update for iMac Graphics 3.0, the firmware update is designed to fix the known issue of iMac Graphics in Mac OS X Lion, which cause users machines to freeze up. The update is released for Mac OS X Lion only, because the issue is only start occurring when users start updating their machines to the latest Mac OS X 10.7.0 / 10.7.1 build.
About iMac Graphic FW Update 3.0
This firmware update fixes a graphics issue that may cause an iMac to hang under certain conditions.
Do not shut off the power on your iMac during this update. Loss of power could result in your iMac failing to start up.
The firmware update weighs in at 482 KB only, and obviously only requires OS X Lion, Apple only releases the update but does not highlight the exactly models of iMac, which requires this firmware update. Therefore now users you have to run the Apple provided Software Update on your machine to look up software update, if it appears then it means you need to update your graphic firmware.