I've connected a 27" LED ACD to my 27" iMac as a second monitor, but the keyboard commands for adjusting brightness do not affect the cinema display. I understand a common cause for this is that the USB isn't connected, but mine is and I can verify it is working because I have an external HD connected to the USB port of the ACD, and the iMac recognizes that it is connected.
Also, if I go into Display Preferences then I do see the Brightness slider for the second monitor. This is currently the only way I can adjust the brightness of that monitor.
27" iMac 2.66GHz i5, Mac OS X (10.6.6), 27" LED Apple Cinema Display