The iPod nano was most definitely announced at a "keynote" presentation. Just because it wasn't an "Expo" doesn't make it any less of a keynote/staged presentation.
I think what parb.johal is getting at is that Apple rarely changes a product significantly without some sort of staged announcement. It's highly unlikely that we'd just happen to visit the Apple store one day and see a completely different form-factor PowreBook -- a completely different PowerBook would most certainly be announced during some sort of staged presentation headed by Steve himself or someone very high up in the company.
It would be nice to have new Powerbooks, with the last G4s before the Intel transition. Hopefully, they will be dual core and the Powerbook 12" will finally ditch the fx 5200 and actually get a decent video chip, like the x300 while the larger Powerbooks go for the x800.