Happn.in is a unique domain name for a unique site. The UI has a list of cites with an Alltop appoach (a la Guy Kawasaki). The top hot topics are listed below each city name. So instead of just clicking a city to see tweets from poeple that say they are there by their location, you can see current trends.
Each city then has a single feed on Twitter you could follow, such as this one for St Louis. So you could keep up quickly. It is based on that Twitter account following people locally which is how I found the service. They followed me recently and I am finally getting them into the catalog. The Twitter account for each city then spits out the current hot trend words, not a major feed for everyone in the city to clarify.
There is currently over 40 major cities listed. I did not see a way to search for other ones so I presume they are adding them in and it would take time to make hundreds, of course.