When I tried to boot it, I got a chime, but nothing else. Every trick I tried, including booting from a CD and setting it up in FireWire target disk mode, failed. Then I tried resetting PRAM, which also didn't work...but it did boot the machine into Open Firmware, so I knew the machine was functional at the lowest level. Typing mac-boot at that point, however, resulted in a scary-sounding error: ALLOC-MEM request too big!. From the sound of it, I thought maybe it was something with the RAM, but I wasn't sure. A quick trip to Google on another Mac found the answer to my problem in this blog post.
It turns out that (at least on this PowerBook, and probably others) the ALLOC-MEM too big! error is caused by a loose AirPort card. You can find instructions on reseating AirPort cards for various PowerBooks in this article on Apple's support site, and the linked blog post contains photos of the process.
After pulling and reseating the AirPort card, my PowerBook G4 started right up just as if nothing had happened -- hooray!

