I just installed Ubuntu 12.04 yesterday and I can't seem to turn the brightness down. The function keys change the slider but not the brightness and the keys work perfectly on windows 7.
I am using HP Pavilion DV6-6121tx notebook pc.
I dont now if this will work for you but I have Ubuntu loaded alongside windows 7 on my Toshiba DX735 all in one desktop. I had to start up in windows to adjust the brightness, shut down and reboot into Ubuntu and the brightness setting held.