After trying to find similar GeoJSON versions of US State and County boundaries, I finally decided to roll my own from the United States [Census Cartographic Boundary Files][1], converting them to GeoJSON and KML formats using the [MyGeoData vector converter][2].

I've made the results of this conversion available at: http://eric.clst.org/Stuff/USGeoJSON. Feel free to use these if they will help.


  [1]: http://www.census.gov/geo/www/cob/index.html
  [2]: http://converter.mygeodata.eu/vector