/proc/acpi/video/...../DOS not available for hotkey display switching

[the]

Hi all,

I would like to switch displays with the Fn+F7 hotkey on my ThinkPad.

ThinkPad: T61
Linux Distro: RHEL5
Kernel version: 2.6.18-87.el5
Graphics card: nVidia Corporation Quadro FX 570M (rev a1).

Examining the Xorg.0.log file I can find this message:

(WW) NVIDIA(0): Error: Unable to find DOS (Enable/Disable output switching)
(WW) NVIDIA(0): file path under /proc/acpi/video. NVIDIA X driver will not
(WW) NVIDIA(0): be able to respond to display change hotkey events.

So I checked /proc/acpi/video and the directory is empty, instead I have a file /proc/acpi/ibm/video but no DOS.

What is missing on my system?

Thank you.