I think the ".in" is perfect for some location specific service. You could build urls like tweet.in/London etc. There are already some services that show you twitter-users in some specific city or area, the problem is that these people do not always tweet about their city, so it's almost useless.

I once thought of a service that lets you add a location to a tweet and a short url. That service would create a map with all your tweets composed in that way.