A firmware update for iMac systems has been released by Apple that addresses problems where iMacs have been hanging during start-up or when waking from sleep. Regrettably which specific iMac models are eligible for this update has not been specified by Apple.
The contents of the updater file show the Flasher utility is for ATI graphics, although this does not narrow the systems down by much because ATI graphics have been used by all recent iMac models except for the early 2009 ones.
Because of the lack of specifics about the eligible models for this update, it is recommend to apply it using Software Update. The update requires OS X 10.6.7 and is a small 700KB download that will install to your system folder and will run the next time you restart your system. Ensure that the update process is not interrupted, which should only take a few minutes to finish. You may see the system boot to a progress bar and then reboot afterward, so follow the onscreen instructions and permit it to run through the complete update process before interacting with your system.