Both your Wikipedia link and your explanation of how entropy enters into the password picture should help kevs understand things a bit better. (The XKCD link was posted earlier by Ira.)
You can get a bit of a "real-world" idea of how entropy works by playing around with OS X's Password Assistant and watching strength ratings change as you change length, characters used, etc, and, as opposed to kevs's linked Web sites, it's right on your own Mac.
