Jon Stokes at Ars has posted his thoughts on the Apple-to-Intel transition and postulates a roadmap for Apple's adoption of x86 chips.
Much of it does make sense, but I'd be surprised if the Mac mini and iBook got the dual-core Yonah. The Mac mini and iBook are low cost machines, and I would expect them to get single-core variants of Yonah to keep costs down. The single-core Yonah parts would be more than sufficient, since even at 1.66 GHz, they are faster than the fastest G4 in current PowerBooks.
I would agree the PowerBooks will get dual-core Yonah chips. However, I am not entirely sure what will happen to the iMac. Like Ars, I would expect it to get a dual-core chip of some sort, but I don't think we can exclude just yet the possibility that it could get a single-core chip, since it is a consumer-oriented machine after all.