Same problem of yours with an hp nx7400 notebook (intel 945gma). xset dpms off shuts down the backlight for a few seconds. Only vbetool dpms off shuts down backlight really. I tried on debian etch (similar packages, same i810 driver, same notebook) the problem is not present.
It's a bug related to ubuntu's Xorg. See also bug: https://bugs.launchpad.net/distros/ubuntu/+source/xserver-xorg-video-i810/+bug/37364 -- Laptop backlight does not turn off https://launchpad.net/bugs/19410 -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
