If you're in the market for a MacBook Pro to replace a PowerBook, you need to remember that Apple's new professional notebook requires a whole new set of accessories.
If you have any PC Cards (i.e. EVDO cards or flash memory readers) you'll need to replace them because the PCMCIA slot from previous PowerBooks has been replaced with an ExpressCard/34 slot.
Memory from previous PowerBooks and iBooks won't work in the new MacBook Pro either because it uses faster DDR2-PC5300 667MHz Low Profile SO-DIMMS. The current PowerBook G4 Aluminum uses DDR2-PC4200 533MHz Low Profile SO-DIMMS. There's a chance the MBP RAM will be backwards compatible with current PowerBooks but forget about using your current PB memory in the MBP.
Whenever I get a new PowerBook, I usually update the OS then shuttle the hard drive from my old PB into the new one. This saves me a ton of time usually spent on migrating data, but that won't work with the MacBook Pro. Previous PowerBooks used Ultra ATA/100 hard drives whereas the MBP has switched to a faster Serial ATA interface. If you had designs on the new Seagate Momentus 5400.3 160GB hard drive for your MBP, you'll have to wait for the serial ATA version.
If you've got any FireWire 800 peripherals, you'll need a cable to go from FireWire 400 (1394a, 6-pin) to FW800 (1394b, 9-pin), but these are quite common and can be found for US$15-20.
Don't forget that the MBP uses an entirely new power adapter, for the first time in five years, called the MagSafe connector. If you've got a few extra Apple AC adapters lying around, you may want to think about ordering an extra MagSafe connector (or three.)
And finally, if you use a sleevecase to protect your PowerBook inside of a bag, you'll need to replace that too because the the MBP is slightly deeper and wider (14.1 x 9.6 x 1.0-inches) than the PowerBook G4 15-inch (13.7 x 9.5 x 1.1-inches). Waterfield Designs is already shipping their MBP sleevecase, so you may want to order one of them now.