It seems that dimming the screen brightness in Toshiba Satellite machines is a big issue for many. I personally have a T130 and I could find no way to reduce the brightness of the screen by using the ' Fn + 3 ' hotkey. In case you have this issue, 2 are the main consequences: 

1) your eyes will start complaining after some minutes you look at your screen
2) on battery power, you won't get much working time

After many hours spent googling around I found the following post on ubuntuforums which allowed me to partially solve my issue:
http://ubuntuforums.org/showthread.php?t=1321403&page=3, post #26

What you have to do is to modify some ACPI configuration files to set the brightness levels you want to use on battery and AC power. In the following, I will try to explain you how to accomplish this.

This procedure has been tested only for Linux Mint 10 Julia x64 Main Edition (i.e. Gnome desktop), 11 Kaya unkown edition, and 13 Maya x64 KDE. In case anybody tests it on a different release/desktop environment, please drop a brief note so I can update the information.

0. Check if SETPCI works on your machine

Before wasting time creating and modifying configuration files, I suggest to test the following simple command in a terminal window: 

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sudo setpci -s 00:02.0 F4.B=45

The command accpets values from 00 (dark) to FF (bright). The instructions should considerably reduce the brightness of your screen. If this is the case, you can proceed and read the following instructions. If not, I am afraid that this Howto will be useless for you  
 

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1. Create the file brightness.sh

The first step is to create a file named brightness.sh in the right place and to make it executable. This can be accomplished by typing in terminal:

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sudo touch /etc/acpi/brightness.sh
sudo chmod 755 /etc/acpi/brightness.sh

2. Edit the file brightness.sh

The second step is to write some meaningful instructions in the file brightness.sh. Open the file with your favourite text editor (mine is gedit) by typing:

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sudo gedit /etc/acpi/brightness.sh

Now copy paste the following lines into brightness.sh:

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#!/bin/sh

if [ "`sed -e "s/.[^ ]* *//" /proc/acpi/ac_adapter/ACAD/state`" = "on-line" ]
        then
                logger "ACPI: AC adapter is on-line, brightness up..."
                sudo setpci -s 00:02.0 F4.B=99

        else
                logger "ACPI: AC adapter is off-line, brightness down..."
                sudo setpci -s 00:02.0 F4.B=70

fi

This will set your brigthness to 99 when working on AC power and to 70 on battery power. You can play with the numbers (from 00 to FF) to find your ideal values.

3. Make ACPI use brightness.sh

The last step is to tell ACPI to use the new brightness.sh configuration file. Type the following lines in terminal:

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sudo su
echo action=/etc/acpi/brightness.sh >> /etc/acpi/events/battery
echo action=/etc/acpi/brightness.sh >> /etc/acpi/events/ac

And finally restart ACPI:

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restart acpid

Now you should be able the screen brightness values on Toshiba Satellite machine and next time you'll turn your machine ACPI will magically remember to adjust your screen brightness. On my T130, the trick works perfectly.

However, you will not be able to manually adjust your brightness by using the Fn key. For this, I haven't found any solution so far.