Well, this is annoying.

I discovered that the boot floppies I was using were built in mid-April;
"make clean" in /usr/src does not descend into /usr/src/distrib, so I had
lots of ancient crud lying around in there.

Unfortunately, the floppy images I downloaded from netbsd.org have other
problems.  sysinst appears to hang at indeterminate points; once it appeared
to hang after starting to extract base.tgz, but it displayed no output while
doing so (I had requested the file display).  The system itself was not hung;
control-C interrupted sysinst and got me to a shell prompt.

Unfortunately, the floppy drive on my main system chose this moment to die,
so I won't be able to make any further attempts until I replace it.  However,
someone else may want to run through the install process with current images
(assuming you've got a spare system you can wipe out).