Why would apple spend a bunch of money developing a new 15" mobo with Bluetooth and what not when the 970 is just down the corner?
From apples perspective, *most* people that want a 15" are either going to buy a 17, live with the current 15, or go to the 12 because of the portability and new features. They won't wait for new 15".
I think the delay of the 17 really screwed apple with their dev dollars, because instead of working on new 17" 970 designs, they had to spend a bunch of money fixing the g4 version, and delaying everything, so now they are going to have to shift some things around, and not work on certain projects, i.e. the 15" update.
(Devil's Advocate : You could look at it that they pushed the release of the 970 powerbook back, because of the problems with the 17" and they will come out with a 15" inch update because the 970 laptop won't be for a while)
I bet apple is putting all of its dev dollars into 970's for all form factors. Its also putting its dev dollars into G4's for the Ibooks.
The 12" Powerbook is to hold over the people that want an ibook but want the Bluetooth features, as well as all the other bells and whistles that the ibook doesn't have, the reason why they need to hold over the ibook people is because the next iteration of the ibook is going to be a G4, and its going to be a whole new form factor as well. I can't say it enough, with the release of a new chip coming soon, apple would never throw dollars at g3 designs.
I honestly think the next time apple updates its laptop line in any significant form it will be across the board, both ibooks, and powerbooks, and the 12" powerbook will be the starting point for the new G4 ibooks.
I wouldn't even be surprised if they hold off completely the rest of this year, and release 2.5ghz desktops, g4 i books, 1.42 g4 imacs, and 1.6 Powerbooks all in san fran in jan.