The CD-ROM drive died in the middle. Oops.
As it turned out, it had installed the OS before croaking, so I was able to shut the system down, replace the drive with one I happened to have sitting around, and bring it back up, then restart the part of the installation that died. It took me most of the day, but it's running now, and everything's just fine. I got an education in OS/400 installation in the process, too.