Very nice and more interesting than the bar charts/graph that other twitter visualization tools give us. It would be really cool (and informative) if you could add a small word cloud when the cursor hovers over a city.

I did a similar non-interactive dataviz a while ago during the big student protests in Quebec this spring. I clustered (using LDA) the tweets talking about the protests and I mapped and identified them.

http://olihb.com/?p=199 (in French, though)