[frog design](http://frogdesign.com) [built a really cool realtime twitter geolocation visualizing app](http://aworldoftweets.frogdesign.com/), that builds a heatmap of locations for tweets posted as it monitors. Here's the heatmap after letting it run for an hour, on a Wednesday, around noon MST:
I'd expected Canada to be much more highly represented - my perception of using Twitter is that it's largely US/Canada/UK centric. Turns out, the US is responsible for about a third of all tweets, followed by the UK, Indonesia (? REALLY! Indonesia. I know!), Netherlands and Mexico. Canada is down in 9th place. Surprising.