American Express launched their new Checkout service today. They also included a full developer RESTful API and authentication via Oauth 2.0, not to mention they are working on integrating with Stripe which already serves thousands of merchants.
Another significant difference is that Amex will pass along customer information to the merchant, if the customer give permission
It is great to see one of the majors launch an innovative product. The challenge will be to compete for critical real estate in checkout pages. Other entrants are Amazon Payments, Google Checkout, and other smaller but no less aggressive processors such as Square and Dwolla.