An application programming interface (API) is now available for Federal Register data from FederalRegister.gov (also known as Federal Register 2.0), according to a 1 August 2011 post by Michael White of the Office of the Federal Register, on the OFR Blog.
According to the post, the API is RESTful, and provides access to metadata and full text data in JSON format. Via the API, data are accessible by agency, by document number, or by advanced search query.
Some comments about the API are available at Hacker News.
For more information, please see the announcement.