well, by taking a 90 nm G5 and clocking it at 1.6 or 1.8 GHz and sticking it into an iMac is possible for 2 reasons.
One, if they go all dual, all 2+ GHz in the PM line there is no problem distinguishing between a PowerMac and iMac. Its a matter of marketing - no matter what happens, there's always some consumer that has problems understanding some part of the 'why' in hardware. For most people familiar enough with hardware specs it sounds silly, I know.
But, the other point is that at lower speeds the CPU will use less power and be cooler. Which could be useful in the iMac enclosure.
Sure, I'd love to see a 2.0 GHz iMac, but that probably won't happen immediately.
D