IMac update 1.3 had nothing at all to do with freezes. That is just an assumption those waiting for a fix had.
The problem with your freezes is a hardware one. The drivers post Imac update 1.0 enable 3d content on your card that some cards will freeze trying to utilize. If it was a driver issue (bad drivers) all imacs would be affected. Take your mac back and get it replaced. That is the only way you will get this fixed. Alot of their ATI 2600 HD cards were faulty that got released. A software update will never fix this unless it disabled the 3D effects of the card and who would want that as a "fix"?
Before you take it in I would backup your data, then erase and install Leopard fresh with a blank user profile. Have it as clean as you can get it so the tech can't argue about 3rd party software with you or about software problems in general.
the best reliable way in leopard to get these to freeze is just to open preferences and goto the screen saver section. Try to view any 3d screen saver such as the word of the day and it will freeze right away.
Don't let them tell you they will give it a load test. A load test will not test or even use the 3d engine on that card and it wont freeze. Just have them try to change the screen saver and then they wont have any choice but to exchange it.
think about it. How could it possibly be a software problem if it is not affecting every Imac with this hardware configuration? The problem only appears to have the characteristics of a software problem because there is no freezing in Imac Update 1.0 and there is freezing in later updates. You need to go deeper then that though and look at what Imac update 1.1 did for the ATI drivers. In Update 1.0 the ATI drivers did not fully support 3d on the card. As a result no freezing. 1.1 and later more fully support the ATI cards 3d abilities. Now it freezes.
Do all Imacs with the ATI 2600 HD freeze? No. Do even a significant percentage? No. How could this then possibly be a software issue? It can't. Given what we know about the trigger for the freezing the only "software" fix is to cripple your video cards 3d. How is this a fix?
There is only one way to fix this. Exchange your imac under warranty or have them replace the defective hardware. The longer you wait to do this the more time you burn from your macs warranty.