Okay, here's a thought, the 970 is debuted in the 15" PB, but, at the same time, the new 970 based PM systems are announced with the shipping dates for these systems put out by a four to six weeks (or something like that). This would give Apple time to get the lower yield, higher speed chips for the PM systems, while still allowing them to release the first 970s in the PBs.
And, I know that this is all pie in the sky right now. I don't necessarily believe that it'll happen. However, I also refuse to see it as an impossibility.
For those who were cited the G4 as an example of how chips always take time to go from the desktop version to the portable version, I'd like to respond by citing the G3. The first G3 desktop systems were introduced in November of 1997. The first G3 PowerBook (aka kanga) was introduced, yup, you guessed it, November of 1997. They rolled them out side-by-side.
Now we have a new chip that should be portable from the get-go, just like the G3 was, and I don't see any real reason why Apple wouldn't release them side by side, or even release the PB version a little ahead of the desktop version (of course, as I said earlier, giving full acknowledgement to the fact that the new PowerMacs were on their way).