A quick fix to the problem of not being able to control the backlight in Gnome 2.24 (Fedora, Ubuntu, etc…) is to run a simple command in the terminal. By no means to I take credit for finding this solution, nor do I promise that this will work for you. It just so happens that it worked for me when my computer would freeze, hang, and crash when trying to adjust the backlight brightness using the “gnome brightness applet”. Also, I’d like to make a note that this was tested in Fedora 10. It should be distribution independent as long as it’s running Gnome 2.24. Here goes:
In the terminal, run this (you may need to run as root):
xrandr --output LVDS --set BACKLIGHT_CONTROL native
Now, if your luck is anything like mine, you should be able to control your backlight using both your function keys (FN+F5 or FN+F6) and gnome brightness applet. Good luck!
Source: Ubuntu Forums