Joe’s onto something. This idea would never work if you had to buy a device specifically for it. Aside from the fact it wouldn’t be cheap I don’t think most people want to carry around more than their cell phone and/or their iPod.
But turning this into an app for a cell phone and using bluetooth would be more effective and also quite a bit cheaper to develop since it’d be all software. Only thing I’m not sure of though, since I don’t have a cell phone w/ bluetooth, don’t you have to verify & allow the two devices first before they can communicate? So maybe that wouldn’t work…