Here's an iPhone app with some potential. Greg Elliott of Poke has released SynchStep, an app for iPhones and iPod touches that chooses songs based on your walking pace. The project began before the iPhone, conceived as a self-contained MP3 player with the necessary hardware. Upon the advent of the accelerometer-toting iPhone/touch, it became more feasible to bring the idea to the masses.
The app has a simple, good-looking interface. I couldn't get it to work very well, though, after about 20 minutes of building and analyzing playlists followed by many carefully paced steps. Quite a few people witnessed me walking quickly by, only to see me pass a while later at a much slower pace, repeated ad
nauseum nauseam with an increasingly frustrated look on my face.
I love the concept, though. I want it to work and I'm hoping I'm just doing something wrong. Maybe you should give it a try. I've always wanted a real-time, personal soundtrack. Coincidentally, the previous name for the project was PersonalSoundtrack, which I mention only because I like the former moniker slightly better for its more intimate connotations. Now, back to walking around like I'm in a Monty Python sketch.Update: After working a little with the Greg (the very helpful developer), I'm happy to report that I do actually have a personal soundtrack now. I had initially assumed that SynchStep would round to the nearest available BPM match, but it's pretty fine-grained. The more songs you have at different BPMs, the more likely you are to get great results. The documentation has been updated to reflect this. Give it a shot, it's impressive.