Comment 22 for bug 1247668

I attach a patch to xorg-server which seems to fix it for me. Side effects are yet to be discovered :)

Rebuild xorg-server with the following series of commands (might not be the best way, but that's what I found, I'm not yet familiar with these):

sudo apt-get install build-essential fakeroot dpkg-dev
mkdir build
cd build
apt-get source xorg-server
sudo apt-get build-dep xorg-server
cd xorg-server-1.15.1
cp ~/Downloads/xorg-server-xkb-numlock.patch debian/patches/ # replace Downloads appropriately
echo xorg-server-xkb-numlock.patch >> debian/patches/series
dpkg-buildpackage -rfakeroot -b
cd ..
sudo dpkg -i xserver-common_1.15.1-0ubuntu2_all.deb xserver-xorg-core_1.15.1-0ubuntu2_amd64.deb