There is a command for Terminal in OS X which allows you to take screenshots from the command line. Creepy creepy. Here is the format:

screencapture -x FileToSaveAs

The -x option tells it not to make the shutter click sound. There is also an -i option which triggers an interactive mode, although I’m not sure why you would use the Terminal to do that.

There is also a cute little tidbit in the man page for screencapture: