Comment 26 for bug 198476

So going through the kernel log at
https://launchpad.net/ubuntu/+source/linux/

This appears to broke in 2.6.24-11.17 with one of the following entries that was pulled from upstream:
  * ACPI: video: Rationalise ACPI backlight implementation <-- "This is most likely the culprit"
  * ACPI: video: Ignore ACPI video devices that aren't present in hardware

This is currently broken in 2.6.25 (which I assume where these patches were pulled from).

Now remember it "works" with 2.6.25-10.16.

So what we know:

     - This effects only Thinkpads with Intel x3100
     - This broke with 2.6.24-11.17 update
     - There is a really bad work around using thinkpad acpi ...
        please read <kernel source>/Documentation/thinkpa-acpi.txt
         It says and I quote .. " When display backlight brightness controls are available
         through the standard ACPI interface, it is best to use it instead of this direct
         ThinkPad-specific interface."
        * So you should not be using backlight= thinkpad_acpi option
        * The problem is the ACPI interface is broken, so please stop mentioning it in this bug
     - Also even though you see the Gnome backlight indicator slowly going up and down, the actual backlight is not changing.