Re-mapping keyboard keys

Some laptop Fn key combinations are multimedia keys (appearing to the OS as ordinary keys with nonstandard codes), while others trigger ACPI events that eventually reach the OS, and some are just handled by the BIOS and never seen by the OS. The information paths are explained on the Hotkeys/Architecture page on the Ubuntu wiki, complete with diagrams.

The KeyTouch program should handle anything that is seen by the OS, and has a friendly GUI for describing your model and configuring what the keys should do (install the keytouch-editor package as well as keytouch).

If the OS can't see anything when you press the brightness keys, it could be because they're handled directly by the BIOS. The battle is not completely lost — it is in principle possible to hack the BIOS — but the difficulty level is considerably raised.