This is interesting:
More than 10 million Americans moved from one county to another during 2008. The map below visualizes those moves. [Click on the image to go to the interactive map at Forbes.com, then: ] Click on any county to see comings and goings: black lines indicate net inward movement, red lines net outward movement.
Source: Internal Revenue Service data. The IRS only reports inter-county moves for more than 10 people, so some moves are not shown on this map.