Sensors and GPS: A perfect storm ?

I learnt today that the iPhone contains an accelerometer. It got me thinking about all the new types of minature sensors that are becoming available, piezoelectric etc and how in conjunction with global positioning satellite technology and wi-fi we can represent all kinds of real-time information on a program like Google Earth. It's getting very close to the scenario painted in Snow Crash, a book by Neal Stephenson in which a program is able to display incredible amounts of location specific real time information. I am sure there is a killer app for all this confluence of information and connectivity in portable devices. I think we will see wireless sports training devices to improve our tennis stroke, football kick and maybe even improve our running gait to reduce risk of injury when running long distances.