-- One of my disks is an old Maxtor LXT-213S that apparently doesn't
       understand the LUN concept.  The -current kernel thinks it's eight drives,
       one at each LUN.

An appropriate entry in scsiconf.c will `fix' that.  There's already:

	{T_DIRECT, T_FIXED,
	 "MAXTOR  ", "LXT-213S SUN0207", "",     SDEV_NOLUNS},

but I presume that doesn't match your drive.

       Fortunately, this didn't get in my way too badly -- it
       did push one of my other drives up to `sd10', after which I was not able
       to mount it (`mount', oddly, complained that the directory I was trying to
       mount on did not exist, when it clearly did) --

Are you sure it wasn't complaining because the device node in /dev
didn't exist?

    -- `mount' doesn't seem to know about the new clean flags -- first `fsck'
       says that a filesystem doesn't need to be checked, and then `mount' says
       that it does.

That certainly doesn't happen on any of *my* machines.  Sounds like
someone biffed the snapshot.