Comment 9 for bug 773471

I have just found a fix for my system. Add this ppa:
https://launchpad.net/~kamalmostafa/+archive/linux-kamal-mjgbacklight
to your sources e.g. with:
sudo apt-add-repository ppa:kamalmostafa/linux-kamal-mjgbacklight
then reload the package information and upgrade your system (e.g. with synaptic)

This should give you a new kernel and gnome-power-manager. Shut down and boot.

For my system this kernel gives me full hotkey backlight control. I hope it works for you too. Let me know how it goes.