Screen dimmer not working toshiba satellite m45

I'm running Ubuntu 10.10, maverick meerkat on a Toshiba satellite M45-S2693.
Since I installed Ubuntu, the screen will not dim, even when running on battery power.
I have "reduce back light brightness" checked under power management, and I have tried the Fn keys, and I have also tried the "Power Manager Brightness Applet 2.32.0" on the task bar. None of these have helped. In addition, the back light applet on the task bar won't even let me slide the thing up and down, implying that the computer knows that it cannot dim the screen.

I would be very grateful if someone could help, I'm losing almost half of my battery life because of this.