An all quad PowerMac lineup would be amazing, but really expensive. Doing this would certainly cut out any chance of BluRay drives in the first batch of Intels (if they want to keep the prices short of astronomic).
Slitting the linup into single core (Mac mini), dual core (iMac), and quad core (PowerMac) has certain marketing advantages that may be more pressing, however. If they do go quad, they might want to look into some sort of SLi setup to really push the graphics up a notch. If you're putting four cores in there you don't want any obvious bottlenecks to take away from its performance.
I just hope the top-of-the-line Intel quad PowerMac is a little cheaper than the G5 quad. Even $100 would be a nice gesture on Apple's part.😀