The point is not that Apple wouldn't KEEP the existing design and up the specs. The point is that if they redesign it, they won't redesign it to look like an old display.
It should also be noted that according to the rumormill that Apple was also much more focused on making the switch in the architechture from PowerPC to Intel, and that after the iPhone and all of their other projects, this is their first chance to really take a shot at updating the design of the current line. So all of the Intel Macs share a relatively close design (some accomodations were made for size and features) to their PowerPC brethren.
I'd have to admit, that having an iMac with the same design as the Cinema Displays may introduce confusion, but it would sure make a second display fit in pretty seamlessly.