Comment 4 for bug 743352

With the Macbook Air, you can take X out of the equation and reproduce the issue directly with the kernel by echoing various values into /sys/class/backlight/mbp_backlight/brightness

With the Nvidia driver active, doing so does nothing just like the brightness keys on the keyboard. Removing the Nvidia driver and rebooting (nomodeset) into the standard vesa xorg setup, both the brightness keys and writing values into the kernel (as above) work again.