IBM T42 Laptop Screen brightness problem

I recently tried changing my screen brightness through the gnome power management tool but found I could not. The fn+home/end keys work fine for brightness except the values are being capped somewhere. When I unplug my laptop the brightness goes down and no amount of (fn+home) will increase the brightness to anywhere near what it was when it was plugged in.

I came across the GnomePowerManager FAQ which has some interesting information http://live.gnome.org/GnomePowerManager/FAQ#head-8cb2644b2021f4d70ed47e3f8843bae6f37a1aa2
but instead of trying /etc/modprobe.conf, I refered to http://kernel-handbook.alioth.debian.org/ch-modules.html
and was about to add 'options ibm_acpi experimental=1' to /etc/modprobe.d/local when I noticed there was already a file called /etc/modprobe.d/ibm_acpi.modprobe which had the following line contained in it
'options ibm_acpi hotkey=enable,0xff9f experimental=1'.

Of course having this already wasn't helping as the brightness slider wasnt enabled in the GnomePowerManager, so I removed this file and went ahead with creating the /etc/modprobe.d/local file with the line from the GnomePowerManager FAQ. I rebooted and still didnt have the power slider. I undid the changes and am now back to square one, any ideas anybody?