Check to make sure your BIOS is up-to-date. It can be that this is because the ACPI on your machine is broken, but I wont rule out a bug in the acpi video driver.
** Changed in: linux-source-2.6.17 (Ubuntu)
Status: Confirmed => Needs Info
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acpi video module prevents brightness adjusting
https://launchpad.net/bugs/74284
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