The Bureau of Economic Analysis (or BEA) has an API. They are the people who come up with the GDP estimates (among other things). You can register for an API key, and read the documentation (their user guide is quite good). The URI is http://www.bea.gov/api/data. You can get data in JSON and XML formats.
Once I become more proficient with Python, I plan on programming a wrapper for this API. I will announce when that project is complete here.