Apple has posted iMac Graphic Firmware Update 2.0 (http://support.apple.com/kb/DL1389). It fixes an issue that in rare cases may cause an iMac to hang during startup or waking from sleep.
Don’t shut off the power on your iMac during this update. Loss of power could result in your iMac failing to start up. The upgrade requires Mac OS X 10.6.7 and is a 699KB download.