If your PowerBook G4 ends up (like mine) booting to a folder with a question mark instead of Mac OS X, and happens that you do not have a System CD with you from which to boot and fix your startup disk, here is a trick that may bring it back to normal (it did for me). It should also work for most new Macs. Boot to Open Firmware by pressing Alt+Command+O+F on boot. At the prompt, type the following two lines:
set-defaults [hit Enter] boot hd:,SystemLibraryCoreServicesBootX [hit Enter]Hopefully your Mac will boot to OS X or (if the trick doesn't work) to a white screen with an error signal instead of the Apple logo. Don't panic, just switch off the box and try the commands again (it actually took three tries for mine to come back). Remember to re-set your startup disk to the Mac OS X folder in the Preferences -> Startup Disk after you get running again!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031119134803664