[ltp] reducing LCD's brightness after inactivity

[Gerrit E.]

Hi,

I would like to reduce my LCD's brightness after a defined period of
inactivity. I know I can do this by hand using die ibm-acpi module but I
don't know how to automate this with Xorg. When the user uses the mouse
or keyboard the LCD should bright up again.
I have seen this functionality in Ubuntu Dapper, but I can't find out
how they did this.
Btw, i'm using Gentoo...

Thanks in advance,
Gerrit