if they ditch the 12" PB/MB as well as the iBook variations i'd think they would have to go w/ a dual core solution. there has always been talk of how the 12" iBook cannibalizes 12" PB sales bc they are so similar, but i don't think it would be smart to eliminate a pro-sumer item like the 12" PB. if they want clear segmentation they'll have a 13.3" widescreen iBook w/ core solo and a 13.3" WS MacBook w/ core duo. however, appleinsider noted that the price difference between solo and duo @ 1.67Ghz is ~$32, so the move to higher processors may open up a slot for a core duo iBook. hell, i could care less. both are sweet chips, so long as they don't celeron the iBook!!!
mind u they could always go w/ a core duo in both models and segment it by way of A/V specs. for example analog audio in/out for iBook while the MB gets digital and or optical. or the MB gets the iSight while the iBook doesn't. aspect ratio could be an easy divider too, 4:3 for iBook and WS for all MacBooks, but really, who knows?