upgrading the root FS to read-write

[perryh at pluto.rain.com]

After booting 8.1-RELEASE single-user, what must I do to upgrade
the root FS mount from read-only to read-write?  I have tried:

# mount -u -o rw /
# mount -u /
# mount -u -w /
and even
# mount -u -w /dev/mirror/gm0a.journal /

I don't get any error messages, but the FS remains read-only.