What date-release of 3.1 are you using? IIRC only the latest or latest two date-releases have mtab fixed so it is "proper" or somewhat proper on a hd install. I think I'm now running 1/1/2003 on my hard drive and the df output is a little goofed up but usable. On my November date-release install I linked /etc/mtab to /proc/mounts and at least was able to see the root filesystem with df. You might look into the FAQ regarding df.

HTH

rock