Switch display Fn4 button doesn work on netbook CQ10-400SD

In Ubuntu 10.4 netbook edition the switch display button of my Compaq CQ10-400SD netbook doesn't switch between local screen, external monitor and both. In fact is doesn't do anything.

Please note that
- on this netbook you use the Fn button to get the function keys F1 thru F12 and you don't use the Fn key to get the netbook specific keys. So the switch display button should work accually without the Fn key. I used Fn4 in the title just for a quick indication of the button.
- other keys work good, like sound -, + and mute;
- An external monitor works fine, and using the "display settings" dialog there are varios options. However the usability is not good:
  - mirroring (same screen) makes no sense, external monitor has different resolution/aspect ratio;
  - not mirroring makes no sense: the Ubuntu panel is on the netbook, however the menu is on the external monitor and applications start on the exernal screen, however the title bar is integrated in the panel on the netbook. Furthermore the option to switch of the netbook screen, simply switches of the netbook screen, without the transfering the panel to the external monitor.

Help will be appriciated.