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.
I have an Intel MacBook Pro.
In my office I have a 30" Cinema Display, wireless keyboard and USB mouse (attached to the display). I have a spare power block next to the display.
In theory I should just arrive, switch on the keyboard, plug everything in to the MacBook(video, usb and firewire connections from the display, power and ethernet from the wall), and go!
In practice, sometimes it worked, and sometimes the MacBook would refuse to talk to the display. Worse still, it would refuse to wake up if I disconnected everything, and tried to run stand-alone. Until I found the secret, I had to force a reboot with the power key.
The secret is, connect the power first!
Connect the power, plug in video, usb, firewire, ethernet. The computer wakes and the display comes to life. Turn on the keyboard, type a few characters (type them somewhere non-critical). A couple of characters may get lost initially, but once the connection is established, everything is fine.
There is a special environment file which loginwindow reads each time a user logs in. The environment file is: ~/.MacOSX/environment.plist
Dot files aren't, by default, visible in the finder, so type cd; open .MacOSX in a terminal. If the environment.plist file is there you can open it in the Property List editor with a doubleClick. Otherwise, you will have to create the .MacOSX directory yourself.
To create the directory and file
$ cd
$ mkdir .MacOSX
$ echo "" > .MacOSX/environment.plist
$ open .MacOSX/environment.plist
This should open your new, empty environment.plist file in the Property List Editor. Select Root and Add a Child: the Key should be the name of the variable you want to set; Type should be String; Value should be the value you want to set for this variable. Next time you log in the variables should be set.
Environment variables set in environment.plist also appear in the standard bash environment. You can see all your environment variables using the shell command env from a Terminal window.
REFERENCE: http://developer.apple.com/qa/qa2001/qa1067.html
WARNING: do not set the DISPLAY variable—see my post re. X11 on Leopard.
