This would be an excellent opportunity to add some encryption to credit card numbers. Imagine taking this, except for it has a pin-pad where you enter in your password/passkey. All credit card numbers stored on the device are encrypted with this key, so it's impossible for a thief to swipe your card. However, it doesn't solve the malicious waiter problem.. but of course, you could still keep it locked down so that the most the waiter could get is one card, rather than all of them.

Ideally, it'd be something like:

* Entering your key keeps exactly one card decrypted for up to 2 minutes * Changing cards requires a reentry of the key

You could even go crazy and do things like allow different cards to have different passkeys. Not sure how useful that would be though