Moonshadow Mobile is making US Census data available on a high performance platform. They have taken the data and geocoded it for Google and Bing maps. The service offering is a proprietary, patent pending system that is able to search through between 20 and 100 million census data records in under a second. Moonshadow has the full census data set, even down to individual voter registration records which only be released to office holders or government entities. The Moonshadow Mobile API offers developers access to a subset of this data.
The Moonshadow Mobile API documentation lists only the attributes available on the request. There are no sign up forms, example API calls, API keys, or pricing terms specified. All those items would make a much clearer picture for the API developer, but it seems like Moonshadow will do well enough without partners. A redistricting application is one of Moonshadow’s products; It runs as a web app. If I had to guess I’d say that Moonshadow is using their own API. Redistricting has been a hot button issue lately, and I'm sure that politicians, news organizations, and citizen activists are all clamoring for a great interface to this data.
Census data is also provided by USA Today. The USA Today data set includes more data beyond what Moonshadow is offering publicly, but the performance is probably not as high.