If you own an iMac with an ATI Radeon graphics card and have been experiencing system hangs (where the computer stops responding), Apple has a firmware update that might help your situation out.
In the past week Apple has issued a number of firmware updates for various Mac systems, including updates for the onboard EFI firmware on its systems to address various problems with network booting and use of HDCP compliant monitor devices, among other issues.
This firmware release is for the graphics card on some iMac systems, and is a small download that includes a ROM flasher for Radeon GPUs. Apple does not mention any details about the specific issues addressed and only claims it fixes situations where the systems may hang.
This update was originally released last August, but it appears Apple has adjusted the update a little, perhaps updating the installer scripts. Both the new and old updates have the same release notes, and both are called "version 3.0," with the difference being the new update was created on February 27.
Apple does not discuss any details on which systems the update applies to; however, if you open Software Update and see the patch listed, then it is applicable to your system. You can also download the patch from the update's Web page, and if upon opening it you see an alert that claims the software is not supported on your system, then the patch does not apply to your computer. If you have already successfully applied the previous release of this update, then you should not need to reapply it.
As always, be sure to back up your system before applying this update, and be sure to follow the onscreen instructions exactly. Connect your computer to a reliable power source, and do not interrupt the update process. It is not uncommon for firmware updates to result in several automatic restarts of your system, so allow the update to run its course when you install it.