Today When I pressed the keys to switch up/down the brightness of my LCD it didn’t respond. Googling around, I’ve found this post https://bbs.archlinux.org/viewtopic.php?id=139610.
The command cat /sys/module/video/parameters/brightness_switch_enabled returned N. So I echoed the opposite echo Y > /sys/module/video/parameters/brightness_switch_enabled. IT WORKED :D Now my brightness keys are working right again. I just put this command on my /etc/rc.local so that it persists on reboot.
Update (05-22-2013):
I just got a Dell XPS 13 that had the same problem but a different file to fix it.
echo 0 | sudo tee /sys/class/backlight/intel_backlight/brightness
This one also might work. Choose a number between 1 and 100 and run the command.
echo -n 6 > /sys/class/backlight/acpi_video0/brightness