After starting to burn a CD with iTunes, I realized it was burning an MP3 CD instead of a regular audio CD. I clicked the 'X' to stop and answered yes to the "Doing this will make the disk useless" dialog. Unfortunately, this left me with a disk in the drive that wasn't mounted, so I couldn't eject it.
I tried the hint posted earlier about using the system profiler to find the BSD name and using disktool to eject it. System Profiler didn't see the disc at all. When I rebooted, it froze at "accessing local drives." I tried booting into open firmware mode, but that also wouldn't work. On a whim, I booted into FireWire target disk mode and hit the eject key. Out came the disc. For reference, I have a 900mhz iBook G4 running 10.3.3.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040528234139319