Tweet-o-Meter [casa.ucl.ac.uk] is an ongoing research project at the UCL Centre for Advanced Spatial Analysis, and attempts to "further the understanding of the social and temporal dynamics of cities within the Twitter demographic".
In practice, this means that an online dashboard-like interface compares the number of tweets (measured in Tweets per Minute or TPM) originating from different cities around the world. Potential questions that could be answered this way include: "Is New York the city that never sleeps? Do Londoners send more Tweets than New Yorkians'? Is Oslo a bigger Tweeter than Munich?", and so on.
However, the more original aspect of this project seems to consist of its attempt to offer this information within the public, urban space. The movie below shows such a small prototype installation of an "analog" dashboard of the same data.
More information is available here.