(CC'ing the Ubuntu NGO group, since they might know more)
This is technology in Linux to do this. After all, laptops do this all
the time. However that's designed for laptops, so it has the concept of
things like "I'm running on battery now", something that desktops don't
Mostly the software would have been written for us rich westerners in
mind and so would always stay on if it's plugged in. It might be
possible to change the gnome-power-manager thingie to turn a desktop
computer off if it hasn't been used in a while.
I don't think Ubuntu comes with anything that could be used for this
purpose; mostly it's just suspending / hibernating or adjusting the
power consumption to the cpu. I'm not 100% sure, but I think you'd
have to build a script, no?
Here's the script. I've tested it on Camarabuntu 9.04. You can adjust
the threshold, I've set it to 20 minutes but you may want to make that
longer - perhaps an hour?
You'll need to copy it to /usr/local/bin/idleout.sh, chown it to
root:root and chmod it to 755.