Apple has acknowledged an iMac freezing issue that was published by AppleInsider earlier this week.
AppleInsider had noted two growing Apple discussion boards filling with complaints regarding the issue (one, two). Indeed, MacRumors' own forums have dealt with the issue.
It has been suggested that the ATI graphics drivers are to blame, as the issue became more pronounced after iMac Software Update 1.1. Both software updates for the new iMac have been geared towards the graphics drivers.
In a statement to Macworld, Apple confirms it is aware of the issue and assures a fix is coming.
We are tracking down the root cause of this bug, and will issue a software update which corrects it as soon as we can most likely later this month, the company spokesperson said. We apologize for the inconvenience.