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.