I lost the functionality of the Fn keys to control brightness and volume after upgrading my Panasonic Toughbook Cf-74 from Ubuntu 7.04 Feisty to 7.10 Gutsy. Turns out that for whatever reason the kernel module which controls the Fn keys is not loading automatically.
To get it working, type the following into a terminal:
sudo modprobe pcc_acpi
That will load the Panasonic acpi driver. And that's it -- your Fn keys should now be working.
To insure that this module loads automatically at next boot, add the above command to /etc/rc.local