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.