Comment 67 for bug 562005

For my future reference, and possibly others:

After installing Xubuntu 14.04 on my Thinkpad W510, generating /etc/X11/xorg.conf with nvidia-settings and adding the EnableBrightnessControl line to the Devices section and rebooting did not work (it worked in Xubuntu 12.10). Instead, I had to create a /usr/share/X11/xorg.conf.d/10-nvidia-brightness.conf file and add the Devices section to that. It's described in this blog post:

http://blog.pearce.org.nz/2012/08/enabling-external-monitor-on-lenovo.html

You should probably change the BoardName to whatever it is for your system (mine is "Quadro FX 880M").