US Population Density Map:
http://www.census.gov/dmd/www/pdf/512popdn.pdf
Persons per square mile:
http://factfinder.census.gov/leg1/04/72954104.gif
Percent Change per county: 2000 - 2006
http://factfinder.census.gov/leg1/11/72873211.gif
Percent Change per State: 2000 - 2006
http://factfinder.census.gov/leg1/03/72955103.gif