Ok.
1.Apple is not going to use a CPU without SSE3 I think, rosetta alraedy makes heavy use of it, dothan is unlikely I think.
2. There will be a single core yonah, maybe they'll use some "celeron yonah" Another reason why i think a powerbook is more likely at macworld, will make full usage of dual core yonah, and having the first dual core laptop will be a major way to announce x86, sounds like Jobs style to me.
3. It'll be hard to make a laptop with an intel processor that's slower than a powerbook, why would you have an ibook that's in some ways better than a powerbook for about 3 months? Why only poeple who are scared of x86 will buy it.
4. powerbook needs a CPU upgrade more than any mac, the ibook and mac mini are consumer products, performance is less important. The powerbook is currently a poor offering for pro users, slow, therefore I don't think it'll be a major issue to move it to x86 first, as many consumers who want a higher end system also buy the powerbook and they'll be happy with x86.
I don't have much faith in this ibook rumor, I think having a powerbook first is both more practical and wiser.