This is more of an FYI than a deep hint. Apparently in Lion, you can now adjust the screen brightness of your Mac even if it's the secondary display in a dual monitor setup.
I'm using an i7 15" MacBook Pro, and before Lion, if my laptop was set as the secondary display, the brightness buttons on my Apple external keyboard didn't work. The brightness buttons only worked if the display was set as the primary display. Now it works if the laptop is a secondary display.
[crarko adds: This is pretty specific. If folks would be kind enough to try a few other configurations and post the results, that could help this prove to be more generally useful.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120114184609800