I've done a little research on this, and I think the way to go is to use a smartcard with an integral LCD display and at least one button.  These are on the market already.



http://www.nidsecurity.com/

Basically, the POS terminal just sends the balance due to your card, which displays it for you.  You then press the button to verify, and the card creates the transaction and signs it.  No need to trust anything.  The card can keep track of your balance, and you can verify it via some other trusted channel just like you do with debit cards now.