I agree with most everything said above, but I'm also in the market for a new PowerBook, and I can think of at least one good reason to wait for Intel: Windows.
For a variety of work-related reasons, I need to run both Windows XP and OS X. At home, it's no big deal: I just set up my Power Mac alongside my XP machine, and use a KVM.
I'm certainly not going to lug two laptops around, however, and so I need a portable that can run both OSs. That leaves me two options: 1) run Virtual PC on the current, PPC PowerBooks; or 2) wait for Intel so that I can run Windows NATIVELY on the PowerBook.
As anyone who's used Virtual PC with the current Macs will attest, emulation entails a significant speed hit.
On the new Intel Macs (from what I've gathered), it should be no problem installing Windows alonside Mac OS X in a dual-boot configuration, and Windows can then run on the machine just as it would on any standard PC. Even if it's not possible to install Windows in this manner, however, we'll still be able to use Windows on the new Intel Macs with Virtual PC, but with one BIG difference: no emulation.
So, for a mobile user with multi-OS needs, it does make sense to consider holding off.