backlight off on lid closure

[Riccardo Mottola <riccardo.mottola%libero.it@localhost>]

Hi,

I have an old thinkpad 600 running 6.1.5 kernel.
I noticed that on closure of the lid (small switch near the display 
hinges) nothing happens, while on my newer T30, the backlight goes off. 
Neither in console nor in X11.
This is quite convenient to save the old lamp or also to leave the 
computer compiling while one sleeps without the light !

Is there software support (e.g. ACPI or some kernel feature) needed for 
htis to work? I see this in the dmesg:

acpilid0 at acpi0 (LID0, PNP0C0D): ACPI Lid Switch

which I suppose is the ACPI device that should trigger this.

What else could be missing not working?

Riccardo