This is an example that makes use of TopoJSON instead of GeoJSON. Once you have included the TopoJSON library, this is pretty similar to working with GeoJSON. You just need to call the topojson.feature function to extract the features that you want, and it will assemble it into GeoJSON format, so the rest of the code is the same.

Since I was getting bored of the US map, I created this map of New Zealand with assistance from Mike Bostock's Let's Make a Map tutorial (which rather explains the styling I used).