This may help somebody. I think my flashing question mark was tied to a problem with external hard drive start-up or removal. I had followed
http://docs.info.apple.com/article.html?artnum=58042 and didn't get rid of the flashing question mark. I could tell I still had a good hard disk by accessing it in target mode (
http://docs.info.apple.com/article.html?artnum=58583). The PowerBook wasn't getting far enough in its boot sequence that it would start up from the install DVD and it wouldn't come up in Safe Mode, or Single User Mode so I could use fsck. It was clueless as to where to find the OS.
Here's what I ended up doing to recover:
1. Start up in Open Firmware by holding down the Command-Option-O-F keys during restart.
2. Type "setenv boot-device hd:,\System\Library\CoreServices\BootX" (yes, those are back slashes) to tell it explicitly where to boot from.
(For the curious, type "printenv" to see all settings. I'm guessing the first column is current settings and the second column is defaults. "\\:tbxi" means "use the boot file on this volume", which doesn't help if it can't be found.)
3. Type "mac-boot" and if you are successful you will boot into Mac OS.
4. Open System Preferences... and set your Startup Disk.
(If you now shutdown, restart into Open Firmware, and type "printenv", you should see the full name of your boot-device, something like pci2/ata-6@D/@0:3,\\:tbxi.)