This is a neat little mash-up.
Say you want to know what people in your geographic area have to say on a particular subject.
It’s trivially easy to find out with this map of tweets by subject.
Or, to be more clear, choose a geographic area using the Google map, and then type in a search term, and it will map out recent tweets that contain your phrase.
Try it out, it’s pretty fun, and could be useful.