Has anyone tried +Swarmly
yet? This is one of the things Google Buzz was trying to achieve with the Buzz layer on Google Maps. The idea is to be able to take a look at a city and get an idea of what's actually happening there.
I'm of the opinion that ideas which seem to recur like this, despite having been tried unsuccessfully before, are generally really good ideas - we either just haven't found a way to make them work so far, or maybe the world isn't quite ready for them yet.