I was able to get my brightness keys working on my Lenovo W530 on Ubuntu 12.04 (posting here to help people looking for answer in future)...
These days X automatically configures itself, so you can't just edit the xorg.conf file, you, instead need to add a section to a file in /usr/share/X11/xorg.conf.d/ and X will include that section in the configuration that it automatically generates.
So to get the screen brightness keys working with your Nvidia graphics card, create a file in the xorg.conf.d directory, e.g:
sudo gedit /usr/share/X11/xorg.conf.d/10-nvidia-brightness.conf
Paste the following into the file:
VendorName "NVIDIA Corporation"
BoardName "Quadro K1000M"
Option "RegistryDwords" "EnableBrightnessControl=1"
Log out and log back in, or reboot, and your brightness keys should now work!
(I blogged this here)