We did the same with Tweets and Surfing. http://devwax.herokuapp.com/ from the meetup: http://www.meetup.com/DevWax/. It was all done in a weekend with some drinking and surfing, so it's a bit rough. The trouble with surfing was that the locations are very disparate and hard to guess. Fun to have a go at though...
Are you in London? (We are)
I'm only using tweets on which users chose to publish their locaiton, so it isn't all of the tweets, but a good chunk of them.
This approach only works when aggregating tweets for a larger area. E.g. comparing 10,000 tweets each in UK county, or perhaps for cities.
For even larger areas (think regions / countries) you could look through the user bios, or previous tweets to pull out any names or locations and do some analysis to work out which broader region they are in.