As reported by ENgadget:
Apple has admitted that at least some of the newest machines do have an issue. Reportedly, an Apple spokesperson stated that a "small number of iMac users have made us aware that a recent software update can sometimes cause their iMacs to freeze, requiring them to restart." Apparently, the machines become "unusable, requiring a hard reset in order to recover," and Cupertino is currently "tracking down the root cause of the bug." ... a software update with [a fix] should be coming "later this month."
This is one reason I usually only sell Apple refurbs to my customers instead of first generation Macs. I've had two customers who bought new iMacs since they came out ... one has had this issue ... the other has had a lot of problems with their keyboard.