The chipset used in the new iMac supports SATA III, and it just needed to be enabled. Hence the firmware update.
As per the current Mac Pro, the SATA ports are on the ICH (I/O Controller Hub) are only SATA II. So the only way to get SATA III in those systems, is to use a PCIe slot. This will change with the next MP revision, but the parts aren't out yet.