If you are running Linux on your MacBookAir and want the keyboard light brightness changed automatically depending on the ambient light (just as OS X does), continue reading.
I’ve written a small daemon named lightum, source is on github and licensed under GPL-2+.
Usage: lightum [-m value] [-p value] [-f] -m 0..255 : maximum brightness value between 1 and 255 (default=255) -p num : number of seconds between light sensor polls (default=8) -f : run in foreground (do not daemonize) -v : verbose mode, useful for debugging with -f
For it to work you need dbus installed, and your MacBook should have the light sensor located in /sys/devices/platform/applesmc.768/light (should be available on all MacBookAir and MacBookPro versions which have a backlight on keyboard, as far as I know).
If you are running Ubuntu, you can install it by adding lightum-mba ppa to your system:
sudo add-apt-repository ppa:poliva/lightum-mba
sudo apt-get update
sudo apt-get install lightum
Otherwise, you can build it from source.