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so let's assess what we think we know:
1. The next powerbook will either be powered by a new Motorola processor, scheduled to be released in Q4 or some variant of the 970 (which hasn't shipped yet in a tower).
2. The powerbooks haven't been updated since Steve declared this to be the year of the powerbook in January.
3. The fact that the 15" is a generation behind on case, firewire, airport, bluetooth suggests it is most ready for an update
The problem is all in point 1 you see. There's no new processor to put in the damn things no matter what supplier Apple chooses. I think it's folly to suggest Apple hasn't invented a motherboard for the powerbook that can take the 970. What Apple is waiting for is for one of these two suppliers to manufacture a damned chip to put in these babies. (I know someone who worked for Apple less than two years ago and saw dual G4 powerbook prototypes back then. These guys have a lot of irons in the fire, and can make unoptimized motherboards pretty much overnight.)
The first supplier to give them silicon determines what goes into the next powerbook. Motorola's unreliability really gives Apple no choice - they'll take a chip from Mot if Mot makes one, but they sure as hell can't put all their eggs in that basket.
You can imagine how frustrating this must be for Apple themselves. There's undoubtedly a big campaign set to go for a powerbook G5, which may or may not be the next powerbook release. Buying media for that campaign is really tough if it happens to come out in fall, because TV is basically sold out already. This isn't complicated strategy, it's simply manufacturing products with supplies you can get your hands on and then doing the best you can to promote it.