Interesting research into figuring out where Twitter users are located, based on similar tweets from other users:

While geotags are the most definitive location information a tweet can have, tweets can also have plenty more salient information: hashtags, FourSquare check-ins, or text references to certain cities or states, to name a few. The authors of the paper created their algorithm by analyzing the content of tweets that did have geotags and then searching for similarities in content in tweets without geotags to assess where they might have originated from. Of a body of 1.5 million tweets, 90 percent were used to train the algorithm, and 10 percent were used to test it.