Often, the simple answers are those you don’t find on the web. Here is one of them with hopefully enough keywords to make it findable:
To activate the Ambient Light Sensor on my new HP EliteBook 8440p (article and pictures will follow later) running Fedora 13, the following command (issued by root obviously) did the trick:

echo 1 > /sys/devices/platform/hp-wmi/als

Also don’t forget to check if the ALS is enabled in the BIOS if this doesn’t work. It seems to be enabled by default though.

HP Ambient Light Sensor on Linux