Comment 13 for bug 74284

Nadav, it's working for you because gnome-power-manager is changing your brightness. If you "sudo killall gnome-power-manager", then you'll not be able to change brightness using hotkeys. However, if you blacklist the acpi module, you'll be able to change brightness using hotkeys even after you kill gnome-power-manager (because Dell laptops don't need any software to do it).