Adjust brightness for 24 inch LED Cinema Display?

I think that the F1/F2 keys on my MacBook Air used to adjust brightness of the primary display. In any event, they certainly don't now (10.5.6) allow me to adjust the brightness of my 24" LED display—they adjust the brightness of the MacBook Air screen, whichever display is selected as Primary.

However, the Display tabs of the Display panels of System Preferences allow you to adjust the brightness of each display independently.