I have an iMac27 run Lion, now I buy a cinema display 27, but at display setting only main screen have brightness adjust controller. And use keyboard to adjust screen brightness also only change main screen.
I search and search, finally find this article https://discussions.apple.com/thread/2649213?start=0&tstart=0 .
Gilletty said that:
So I plug cinema display usb to iMac27, and reopen display setting, now cinema display has it own brightness controller.
It's way simpler than that guys. The brightness on the display is controlled by simply pressing:
Control+F1 OR Control+F2 to adjust brightness on your secondary screen.
F1 and F2 to adjust brighness on your main screen.
If you have adjusted settings in System Preferences for the Function Keys to behave as standard Function keys, you will need to hold the Fn key in addition to each combination too.