I'm starting to wonder if the iMac freezing issue isn't actually a hardware problem. It seems like a lot of the early aluminum iMacs were affected by this issue. Aluminum iMacs that have shipped in recent weeks seem much more stable. Mine has yet to lock up after a week of hard use.
Maybe some of the early Aluminum iMacs got a bad batch of Radeon 2600s? It wouldn't be unprecedented, seeing as the original Radeon X1900XTs that shipped with the Mac Pro were faulty.
Got mine Aug 22nd. No freezing