Elliot, if you use the Nike plus GPS app which you can get on an iPhone 4s, you don't need a sensor anyway so can wear any shoe you want. The iPhone has GPS built in so I think you only need the sensor if you are just using an iPod nano.
The sensor is still £15-£20 and can not be recharged and the battery can't be changed. Bit of a rip off.
I think that the Nike + compatible shoes have a little compartment under the insole where you put a sensor. I do not use Nike + app but do use the compartment to put the foot pod sensor for my Garmin as it fits perfectly.