I was recently documenting some OS X laptop features using screen grabs (Command-Shift-3 for full-screen, Command-Shift-4 for regions). I noticed that when taking a full screen grab when I had a second display connected (in non-mirrored mode), the resulting PDF would be two pages -- each page held the grab from one screen. Very useful.
[robg adds: If you ever don't want both screens, there's not really a one-click solution -- your best bet is to use Shift-Command-4 to drag a region, and just drag the one display you want.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050119085610336