Comment 14 for bug 29767

Can you:

  sudo tail -0f /var/log/acpid

while pressing Fn-F12 and also paste the output of:

  cat /proc/acpi/ibm/hotkey

then if you're seeing the event, check that it is making it as far as HAL with:

  lshal -m

and if it's getting that far, this is maybe a gnome-power-manager issue.