The debit card shouldn't require any trust. The card itself should be able to sign a transaction, transmit it to the card scanner, and have the scanner propagat

The debit card shouldn't require any trust. The card itself should be able to sign a transaction, transmit it to the card scanner, and have the scanner propagate it to the network. Obviously, involving a third party exchange is the only way (right now) to spend BitCoin in places that only accept fiat, but at the very least on BTC -> BTC transactions should be direct and not through third parties.