I've been running with my iPod for a couple years, but put off getting the Nike + iPod system at first since I don't like Nike shoes.  Well it turns out, you don't need the shoes!

There are two parts - the sensor, which fits inside any pair of compatible Nike shoes and the receiver, which is a small adapter that plugs into your iPod nano.  If you don't have Nike shoes, don't worry - you can just duct tape the sensor to your shoe or pick up any number of third party clips that sell for $5-10.

Once you're set up, you can calibrate the sensor of just start running.  For me, it was decently accurate...