The iMac (mine actually) has been shown on one Geit@home this year and another in 2011, so thats atleast 2.5 years. I also kinda remember Frank posting a screenshot somewhat shortly after bigfoot got the G5 tower booting 1st time in late 2010.
So yes, initial iMac port was done long before SAM460 ever got mentioned.
As to supporting R600s, one has to ask wether this is a just a simple framebuffer which is rather trivial compared with a proper driver.
Other than those rare (and still unsupported) R600s those PCIe-PMacs offer very little compared to the faster AGP models.
