I believe we are going to be getting a new powerbook 15.4" very soon, we think it is being manufactured in Taiwan now although Apple do not seem to have let on.
It makes sense really as the 12" is to small for a lot of people to use and the 17" is really an oddity as it isn't that portable. As well as the current 15" Powerbook still being a titanium shell being very easily broken I dunno about the specs, I like to speculate it could be a G5 (I'm G5 on the brain atm) but to be fair it will probably be a quicker mobile G4 rather than a total lab burner It should be made out the same material as the 12" as well to be stronger and give better airport/wireless reception.