According to
this article on gizmag a company called Emue Technologies has developed a credit card that uses eInk as a security feature.
The way I understand it, the credit card has a keypad in which you enter a PIN and the card generates a unique number that is displayed with the eInk and is good for that transaction only. You use the transaction number like you would the 3 to 4 digit security number currently found on credit cards. Not all of the infrastructure is in place for these cards to be viable, but I thought the concept and the use of eInk was interesting.