Apple has just released iMac Graphic Firmware Update 3.0, a brand new firmware update that aims to address an issue where some user’s Mac’s would hang/freeze up under OS X Lion.
The firmware update must not be interrupted once it is started, as losing power for any reason could result in your iMac no longer being able to start up. The update is small, weighing in at just 482 KB and requires OS X Lion.
Apple does not indicate which iMac models actually need this update, so all iMac users should run Software Update to see if this is available for your machine.