Apple has released updated firmware for iMac computers running Mac OS X 10.7 Lion.
Here are the details:
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.
My guess is that this update addresses the system hangs that some I ac users running Lion have been reporting.