Apple Nike
Apple are being sued by a US company called PhatRat who say the Apple Nike Kit was based on one of their products. Apple teamed up with Nike to produce Apple-Nike which relays the rate of your steps to your iPod which selects suitable tracks to keep your workout in tune with your personal sountrack. PhatRat make various performance measuring devices.
A radio transmitter placed in your Nike trainers sends signals to a special receiver attached to your iPod (I assume you have to select what tracks are suitable for what level of exertion, unless trainers now have music taste!)
To use this system you will need Apple Nike kit (£19), Nike+ trainers and an iPod.