The carrier-backed Isis mobile payment service added a batch of new financial partners earlier this year, and it's now back with another fairly big addition. The venture announced today that it has brought American Express' various credit card offerings into the fold (including the company's own mobile-linked Serve cards), which will let users of those cards load them into the Isis app and make payments with their NFC-equipped smartphone -- when the service begins its rollout this summer, that is. As you may recall, American Express itself had already signed on as an Isis partner last year, but this is the first time that the
