Super cool, but my guess is that there's a very small time window for it to succeed before the phone completely subsumes all card functionality. After all, the phone already scans the old card and sends the details to the 'CoinCard' (and monitors card-custody) via Bluetooth-LE.

It won't be long before most swipe terminals are themselves augmented with wireless transceivers, making the "CoinCard" a redundant middleman-device. That is, your phone could just send archived magstripe details, after your onscreen-app confirmation of payment-intent, to the retailer's terminal.

Coin's real strategy may be for that world - the hardware will fall away like a first-stage-rocket at some point... even faster, say, than Netflix moved from DVDs-through-mail to pure-network-delivery.