Update 10:50 a.m.: Apple confirmed that the iMac patch corrects the freezing issue reported by some users, and it's encouraging people to download that patch as soon as they get a chance. Also, the Macbook Pro patch is just for Tiger users; the graphics stability issues fixed by the patch were corrected in Leopard.
Apple didn't provide any details on what the patches correct, but it calls them "important bug fixes," so they're probably important. MacFixIt and AppleInsider said the update fixes the graphics problems that caused iMacs to freeze. The patches are for the latest iMacs introduced in August, and there are separate versions for those running Tiger (Mac OS X 10.4) and Leopard (Mac OS X 10.5).
There's also a Macbook Pro update that is said to improve "graphics stability" for owners that are using the 2.2GHz or 2.4GHz Core 2 Duo chips. There's only one patch, and it's not clear whether that applies to Tiger or Leopard users. I'd assume it applies to everyone, but I e-mailed Apple for clarification just in case.
The patches are available here on Apple's support site, but you can also find them by clicking the "Software Update" menu choice under the Apple menu. They might also be waiting for you the next time you boot.