I was recently sent a link to Urban Challenge, a public transport treasure hunt in the city. People race around the city with a sheet of clues and a camera phone and have to take photos as evidence of their results in the correct order. There is about $50,000 for the winners of the many heats scheduled all through the U.S until November. I was disappointed in the lack of any other location based wireless applications but it's a simple system and would still be fun. Given my propensity these days towards public transport, I'd like to see this challenge in action in London.