Interestingly, Mac OS X Location Services are only available if you have a wireless connection enabled. I would've expected Apple to default to your external IP if it couldn't determine your location based on your wireless connection, but it doesn't.
Most people probably already know where they are but this tool is surprisingly useful.
[crarko adds: I tested this, and it works as described. It asks for permission to find your location just like an iOS app would.]

