BenS sent me this link, which appeared recently on the Forbes website. It is fascinating.
For each county in the US it shows where people in that county migrated from and where they migrated to (within the US) during 2008 using IRS data.
2008, eh? I wonder what it would look like for 2010 as people continue to adjust to the bursting of the housing bubble. But even in 2008, look at the out-migration from Los Angeles county in California or Wayne county (Detroit) Michigan and contrast those with the heavy in-migration to Maricopa County in Arizona or to Harris County in Texas. And particularly intriguing is the migration from the northeast to Miami-Dade County and the simultaneous out migration from Miami-Dade County to the west coast.
Have fun! I had to force myself to stop playing with it.