My macbook pro (running linux) has a strange issue where I have to run setpci -v -H1 -s 00:01.00 BRIDGE_CONTROL=0 before the backlight works (when using official NVIDIA drivers, nouveau works fine).
My T410 thinkpad required some XOrg.conf setting get the backlight working. It really is strange how bad the NVIDIA drivers are with this backlight stuff.