2005-2009 American Community Survey County-to-County Migration Files
http://www.census.gov/newsroom/releases/archives/american_community_survey_acs/cb12-51.html
Press release which discusses the release of data files which measure the flow of people migrating from one part of the U.S. to another and includes links to the data. Provides tables for each county in the nation showing both inflows and outflows over the five year period. From the Census Bureau.
