Apple has released a firmware update for iMac systems that addresses problems where iMacs have been hanging during start-up or when waking from sleep. Unfortunately Apple has not specified which specific iMac models are eligible for this update. The contents of the updater file show the Flasher utility is for ATI graphics, though this does not narrow the systems down by much because most-recent iMac models except for the early 2009 ones have used ATI graphics.
Because of the lack of specifics about the eligible models for this update, even though the updater is available as a standalone installer, we recommend you 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. Be sure you do not interrupt the update process, which should only take a few minutes to complete. You may see the system boot to a progress bar and then reboot afterward, so follow the onscreen instructions and allow it to run through the complete update process before interacting with your system.
As always, before applying this update be sure to fully back up your system. Use Time Machine or a cloning utility to make a restorable or bootable backup of your hard drive, so if a problem happens and you cannot boot your system then you will be able to restore your drive to another system, or at least boot to it or access files off it from another system.