Motorola made a presentation that mentions the dual-core G4 at their Smart Networks Developer Forum held in June 2003 at Disneyland Paris (Motorola also posted PDF file on their website from the presention listing the dual-core G4, which the company has subsequently removed from their website. Perhaps partially because of an Apple objection to it being displayed).
Here's the link (unfortunately registration is required to view the article):
http://www.pcpro.co.uk/news/news_story.php?id=42928
Notice that in the presentation Motorola mentions 'process and architecture improvements' that will bring the PowerPC above 3GHz. That would likely come in the next chip model right after the dual-core since another process would only move the existing G4 architecture from 2 GHz to about 2.6 GHz. A jump to 3 GHz is simply to great a leap from a process shrink without some significant chip architecture improvements. So it seems that speed improvements to the current G4 architecture will come to an end when the dual-core comes out (again probably around July of this year).
Concerning the use of a G5 in the Powerbook...
Apple could use the 970fx in a PowerBook up to a maximum of about 2 GHz, since it uses about 25 watts according to IBM. However, the next version of 9XX chip ,which will be based on the Power5, will almost certainly use more watts than the 970fx at the same frequency. That's due to IBM adding simultaneous multitasking, which is similar to Intel's Hyperthreading. That leaves Apple with using the 970fx in the PowerBook in the next few months and waiting about a year after that for a process shrink to arrive in order to increase the speed of the PowerBooks, or else IBM is working on a PowerPC chip that would suit Apple's notebook use more ideally. Another PowerPC chip in the works by IBM for Apple is purported to be the case according to a recent report from another rumour site.
Motorola is expected to make the Freescale spin off of the chip division in the summer, and yes, the PPC G4 would be part of that. However, that should not stop or delay the dual-core G4 from being produced.