This bug has been around for a long time, crossing several GNOME releases and Ubuntu releases, at least for my laptop. I finally find a workaround: use the default ACPI handling to deal with the lid close instead of using GNOME power manager. To do this, open /etc/acpi/lid.sh with root privilege. Find this line:
if [ `CheckPolicy` = 0 ]; then exit; fiThis line checks if GNOME or KDE power manager is running. If one is running then the script will simply exit to let the power manager to handle the lid close event. So what we need to do is to comment this line out, to let this ACPI script properly turn off the backlight (and also light it up on lid open). Remember to save the modification. Next step is to let GNOME power manager do nothing on lid close, which can be done in Ubuntu by the menu System -> Preferneces -> Power Management. In case the "do nothing" option doesn't appear, like my laptop, use the Configuration Editor to do it: execute gconf-editor; go to /apps/gnome-power-manager/buttons, change the values of both lid_ac and lid_battery to "nothing".
