So it was the graphics card after all!

Apple’s released iMac Graphics Firmware Update 1.0, which specifically fixes the freezing issue of the new iMacs, which it turns out is directly related to firmware issues with the ATI Radeon HD 2600 PRO video hardware in them.

The freezing has been a long-standing issue that had people scratching their heads because recent updates purported to fix the problem but actually did nothing. Users of the new iMacs had their computers just lock up on them at random intervals, with no seeming rhyme, reason or recurring circumstances. People have speculated that it was a video issue, and this update confirms it. Apple says

This update is for 20-inch and 24-inch aluminum iMac computers with the ATI Radeon HD 2600 PRO graphics processor, running Mac OS X 10.4.10 with iMac Software Update 1.2.1 for Tiger, or Mac OS X 10.5 with iMac Software Update 1.3 for Leopard.”

So go and thaw out your new iMacs by running Software Update.