There are a few companies around that provide geo tracking services. Tried geobutton myself before they switched to become a pay service. You can use these services to offer region specific content or to just surprise your visitors (like in your case).
Birdman
10:01 am on Jan 6, 2003 (gmt 0)
I have webtrends stats on a site and it gives stats on countries, states, and cities that users came from. I can't answer the question "how", though.