I think Apple just doesn't want to piss off Motorola while they are still invested in their G4. The two CPUs that do/will come from IBM are the G3 and the G5. I remember hearing a while back that IBM could really push ahead with the G3 in speed, except Mot couldn't pull through with their speeds and thus you don't want to market a lower performance CPU with higher clock rates even if it doesn't perform faster. A lot of buyers just look at the numbers, 800 versus 900 versus 1000, etc.
IBM is a MUCH, MUCH more driven company than Mot ever seems to have been, at least in the computing area. And I think it would be possible to launch a G5 Powerbook by sometime mid-late October and be shipping by sometime in December. I don't think the heat factor is as huge a problem as we think if used in a portable-type design. Lets just look at the desktop, I'd imagine the most heat comes from the drives, high-performance graphics card options, dual CPUs and the interconnect system, and 8-RAM slots with large capacity modules. A powerbook would use a lower-energy system, 2 RAM slots capable of 512MB-1GB each, and a slightly less agressive graphics system, not to mention the portable drives will produce less heat as well.
I think we are overplaying the heat factor and stepping aside the marketing standpoint. Apple needs to play catch up to what buyers are used to with the PC machines to stay competitive. They have done this starting with the PMG5 and will move next to the PB, then on to the iMac while likely bringing the iBook to a G4 or something similar ASAP making a complete transition away from Mot by the end of 2004. Mot has been dragging them down with less capabilities and drive to move Apple's products forward which help the industry innovate. It takes competition to innovate less we think of that also.
On a last side note, with the scuffle going on over IBM and the AIX OS they use and call their own, too bad Apple and IBM don't get together to produce a killer combo, IBM technology, Apple OS that's has a great & easy OS for all to use, you don't have to be trained to use it. They could use an Apple-like hardware spec on the low-end, with full power, POWER4 machines up the line making great cost effectiveness. Also, many of the great engineering apps available for AIX would potentially be ported and make the availability to Mac users, oh and you could even run those powerful apps in a portable environment, can't do that currently with any other UNIX-type machine other than Apple OS X machines and one Sparc portable I think made by Tadpole. Just a little idea that would be awesome to be realised. I wouldn't want to see them lose their Windows machines though as the XP/2000/NT-type market is big and IBM is a respected player there. It would be the best of both worlds.
BTW, I am holding out with my iMacG4-800-Superdrive until the G5 Powerbook is out also. Ordering as soon as it's released. I think Apple will see the urgency with hopefully a renewed interest in the platform with the new PMG5.