The button that is supposed to toggle the ambient light sensor on HP EliteBooks 8440p and 8460p per default switches off the touch pad. That is because Fn+F11 (scancode 0x33), which is the ambient light button, is incorrectly mapped to KEY_TOUCHPAD_OFF (keycode 193) in kernel.

To remap the scancode to another keycode, you can use setkeycodes. An appropriate choice for a target keycode could be KEY_PROG1 (XF86Launch1, keycode 148) which can be easily reconfigured in KDE or any other desktop environment to run any custom command.

Run the following command as root to remap Fn+F11 to KEY_PROG1:

# setkeycodes e033 148

Resources:
http://www.linlap.com/hp_elitebook_8460p