Thanks Jeff, but my problem is that vi won't even start.  Neither 
   will ex (of course) or ed.  It seems to be that /tmp is read only.  
   the permissions don't reflect this.... so I dunno....

Are you in single-user mode?  What does the df command say?  If it
indicates that the "root_device" is mounted, you must mount the "real"
filesystems on top.  For example, if you /etc/fstab is set correctly,
the command mount -a -t ffs will do that for you.  Otherwise, you'll
have to mount what you want indivdiually.

Cheers,
Brook