So with the room lights off, after hitting F10 to bring the keyboard backlight up, F9 does not bring it down?
One more thing to try is System Preferences > Keyboard & Mouse > Keyboard Shortcuts tab and click the Restore Defaults button, in case some remapping was done.
If the answer to the first question is yes, and the suggestion above doesn't help, it's starting to sound like a hardware problem.
You can test that by doing the following: System Preferences > Keyboard & Mouse > Keyboard tab and check the box to "Use F1...". Then, go to System Preferences > International > Input Menu tab, and check the box next to Keyboard Viewer. A flag will appear in the menu bar, click that and select Show Keyboard Viewer, which brings up a keyboard image that shows keystrokes. See if the Fkeys flash when pressed (note that with the Use F1... box unchecked, they won't flash).