FWIW I've got the same issue for one Ubuntu install, but with Xfce. I get this for Ubuntu Precise, but somebody at Ubuntu Studio mailing list had this issue some releases earlier. IIRC somebody mentioned to set it by xorg.conf. I'm not trying to solve this issue on my machine, since it's not urgent, but ASAP I try to disable it by xorg.conf, since I anyway use xorg.conf for all my installs.