I’ve been annoyed by the behavior of recent Debian/XFCE4/GNOME3 releases that have reduced the number of screen brightness levels from the T60′s default of 8 to 4. The problem seems to stem from the removal of hal from recent desktop standards development. I posted this question to a couple of related mailing lists and learned the above from Debian bug report 627336 and XFCE bug report 7541.
The work-around is to disable XFCE’s handling of the screen brightness hotkeys as follows:
xfconf-query -c xfce4-power-manager -n -t bool -p /xfce4-power-manager/change-brightness-on-key-events -s false
xfconf-query -c xfce4-power-manager -n -t bool -p /xfce4-power-manager/show-brightness-popup -s false
The first command disables XFCE’s handling of the hotkeys and the second disables XFCE’s On Screen Display.