My question was why do I see the "file system not clean" warning
   (from sys/ufs/ffs/ffs_vfsops.c) when I'm simply changing the mounted
   partition from read/write to readonly (the "-u" flag to mount(8)).
   without unmounting it first. I would expect the switch to readonly
   to have flushed any pending writes, and then switch the filesystem
   to readonly mode.
   
   The mount(8) manpage says this about the "-u" flag:
   
   > -u      The -u flag indicates that the status of an already mounted file
   >         system should be changed.  Any of the options discussed above
   >         (the -o option) may be changed; also a file system can be changed
   >         from read-only to read-write or vice versa.  An attempt to change
   >         from read-write to read-only will fail if any files on the
   >         filesystem are currently open for writing unless the -f flag is
   >         also specified. ...
   
   If I do the following:
   
   	# newfs /dev/rwd0c
   	# mount /dev/wd0c /mnt
   	# mount -u -o ro /dev/wd0c /mnt
   	# 
   
   I do not get the warning.
   
   If I change something on the filesystem before I do the "mount -u"
   to switch the fs to readonly, I get the warning:
   
   	# newfs /dev/rwd0
   	# mount /dev/wd0c /mnt
   	# echo foo > /mnt/foo
   	# mount -o ro /dev/wd0c /mnt
   	/dev/wd0c: file system not clean (fs_flags=1); please fsck(8)
   	# 
   
looks like you forgot the "-u" here?  this is mounting wd0c again.