Adobe Cirrus (formerly Status) is the technology that Chatroulette and most clones are built on. Chatroulette was not much more than a slightly modified example program provided by Adobe for Status.

Cirrus sets up the connection between the two clients and then the clients talk directly to each other. Since there is a direct connection to your 'anonymous' client you can get the IP and then geolocate. Bing. Bang. Boom.