Apple has released iMac Graphic FW Update 3.0 to address graphical hanging issues that some people were reporting on iMacs. After upgrading to Lion, many users had reported their iMacs were locking up entirely after playing certain kinds of videos, requiring them to reboot in order to regain control. This firmware update hopes to address this problem.
As with all firmware updates, the file itself is small -- 482 KB -- but it does require you to reboot your iMac and ensure the power is not interrupted during the update. Let us know in the comments if this update resolves your iMac's issues.