Have you looked in the bios, (setup)?
I know it's a little inconvenient not being able to do it on the fly when the operating system is running, that's how I change it on my Dell XPS. I can dim it a lot more if I remember correctly, for those times I need to be inconspicuous.
If your using Gnome desktop, also have a look in the gnome-power-manager for a brightness slider.
And my last suggestion:
Try Googling "laptop hotkeys in Fedora 8" or similar. Here's
one that might help.