It could be said that the difference between the MacBook and the MacBook Pro isn't as big as it used to be. Also, I basically got the "20-inch iMac = MacBook and 24-inch iMac = MacBook Pro" idea when I considered the possibility that the 20" iMac will not be able to handle the 65 W quad-cores. So I can also see the 20" being limited to mobile dual-cores while the 24" gets quad-cores, a significant differentiation point. The distinction would really be noticeable if the 20" gets the 9400M and the 24" gets the 9600M GT. But the high-end 20" has the 8600M right now, so maybe the new high-end 20" will also get the 9600M, and so the differentiation won't be as much as with the notebooks.I could see them doing that potentially, but there would have to be a little more than that to differentiate them. If they would really bill it as you described, I would think more noticable of a difference would be in order... guess we'll find out soon (I hope, for all your sakes... I will not be in the market for another 2-3 years, I hope...)
An interesting thing I just thought of is that the high-end MacBook and low-end MacBook Pro have the same CPU. So even if the two sizes of iMac diverge, they would likely diverge in a different way from the notebooks.