Forbes has an interesting interactive map mashup showing where people are moving to and from in the USA.   I have spent way too much time interacting with it. This is a screen shot.

Here is the link to go live and  interactive.  Click on any county to see its comings and goings.  Black lines represent inbound movement; redlines, outbound movement.

Forbes states that  more than 10 million Americans moved from one county to another during 2008.  The maps are visual representations of those moves.

Source: Internal Revenue Service data. The IRS only reports inter-county moves for more than 10 people, so some moves are not shown on the maps.