Spreedly has just made support for JSONP publicly available for adding payment methods. While we have had support for CORS for some time now, the addition of JSONP improves options for developers. The transparent redirect is still a great places to get started for adding payment methods with Spreedly.
Rest assured that XML will remain fully supported. If you choose to do mobile development in a native language with a networking library, you can choose to directly add payment methods. It is an endpoint that is friendly to JSON that you can work with directly for creating payment method tokens. With all of these options to choose from, it can take a little bit of analysis to choose which one is best for your needs.
Our adding payment methods page outlines the different options and assists you in making a decision.