My 24" was ordered on 8/7 (release date) and manufactured between 8/13-8/19. I have not had any freezing issues at all to date in either Tiger or now in Leopard. I've tried doing some of the things that some experiencing this problem say cause their iMacs to freeze such as switching around the menus in Front Row, etc and running screensavers and no freezing, no artifacting, no problems.
I installed the 1.3 iMac Update because I always install all driver/firmware updates that Apple releases and can notice no changes at all.
I know this problem exists for quite a few folks and I AM extremely sympathetic and DO care. I still wonder if this is a software or a hardware issue though. Maybe we should start comparing what software we are all running on our systems to see if there might be some common factor on those WITH the problem as opposed to those without.
I'd be glad to help any way I can. I can understand how frustrating it must be.
I ordered this machine with 1x1GB of RAM and have upgraded it to 2GB with 1GB I purchased a while back and had been using in my previous 20" Core Duo iMac.