The Balanced iOS SDK by Ben Mills allows API communication with the aim to implement credit card and bank accounts into applications. Usage includes code to create a marketplace project, create a card, and create a bank account object.
If you know how to develop mashups then you may be in line to win some very big prizes. To get a sense of just how much money and how many prizes, take a look at the ProgrammableWeb Contest Guide and you'll find that there have been more than 45 mashup contests thus far.
Dwolla has launched a number of services and APIs in the recent past in an attempt to achieve its goal of "building the ideal API to move money." In an attempt to streamline its various services, Dwolla has repositioned its Access API to act as the umbrella to house all of its payment services.