i installed xubuntu 9.10 on my inspiron 11z.
pressing on brightness up/down keys shows notification as if changes are being made to the screen's brightness. but i don't see any change in actual brightness.
Do you have any BIOS settings for the brightness or power settings for the brightness? (I.e. when on AC power always show max brightness)
I'm running Ubuntu 9.10 on my Inspiron 640m and the keys work as expected.