I can confirm that this still happens on 12.04 LTS. Adding the line to xorg.conf as suggested by Pierre Chifflier makes it work correctly. I'm using proprietary drivers too.
-- You received this bug notification because you are a member of Ubuntu Desktop Bugs, which is subscribed to gnome-power-manager in Ubuntu. https://bugs.launchpad.net/bugs/762670 Title: Laptop brightness on Dell XPS not controllable To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gnome-power-manager/+bug/762670/+subscriptions -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
