Merging Census API data with map layers

To the best of my knowledge, the Census API doesn't do any GIS data.

Our CensusReporter.org project has done some experimental stuff which you might find useful. We have a general purpose API (which is the foundation of our own site) which is documented at github.com/.../census-api We have API endpoints which can return GeoJSON and attribute data, although if you make requests for too many geographies, it may not go well. Performance tuning is an ongoing initiative.

We also have code which works with Leaflet.js to make it easy to add specific Census geographies to your map: censusreporter.tumblr.com/.../more-ways-to-use-census-boundaries-on-your-slippy You should be able to use this with the Census Bureau's own API to do thematic mapping.