From HP EliteBook 8540pIf your brightness control works and just need the Fn Key then you don't need the HAL fix below.

The fn+f9,f10 keys (screen brightness/backlight) can be enabled by editing /usr/share/hal/fdi/information/10freedesktop/10-laptop-panel-hardware.fdi at line 27 so that it reads:
match key=”/org/freedesktop/Hal/devices/computer:system.hardware.product” contains_outof=“8540p;4410s;4415s;4416s;Compaq 5×5”
then reboot.