My Fn brightness suddenly stopped working on my Thinkpad T510. I traced the problem back to the Nvidia section of my xorg.conf file. After the following small edit, the brightness keys worked again.

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    Option         "RegistryDwords" "EnableBrightnessControl=1"
EndSection