I have a web app in C# - a simple query tool - that can now serialize the JSON result set for Decennial data into XML. By joining that Xml document to the Concepts metadata from the apivariables.xml, the app allows you to query for up to 4 datasets, and put criteria based on state and county, and to specify a minimum and maximum value filter for the result set. It will display the states and counties that match your criteria, and the description of the Concept of the dataset, plus a record count, and the sum of the records returned. I'm still trying to figure out how to turn this application into something more practical...currently I'm thinking that if a certain population (e.g. Lumbee) reaches a certain threshold in a particular geographic area (e.g. North Carolina), the data will be somehow mashed with data from the Google API to return the closest ethnic restaurants for that population. (Hey, I was hungry when I began this application and that was the first idea that popped into my head).
The app can be downloaded from https://github.com/nickcamporillo/Census_Api and the file is Census_API_WebApp_01_28_2013.zip
Also, please use your API key by going to the web.config file under the node "-censusApiContext - censusApiConfig - Settings" and enter your key there in the "Value" attribute. Thanks.