When my MBA is hooked up to the older Aluminum 20" ACD, the apple bluetooth keyboard buttons for brightness do not change the brightness on the monitor.
I've found out if you plug in the USB cord from the monitor into your Mac, you should now have brightness controls working from the keyboard!
I have a 2013 MBA with the Apple BT keyboard but always ran the display with just the minidisplay port to DVI adaptor plugged in to the Air leaving the USB and FW cables dangling. But when I also plugged in the USB cord, now the keys work to change the brightness.
You also will get the option of putting your computer to sleep when hitting the display power button or just turning off the display, too. Check System Preferences --> Displays --> Options