Forbes.com has a fascinating interactive map
up that uses IRS data from 2008 to show the migration of Americans on a county-to-county basis. Above is that data for Miami-Dade. Red lines represent migration patterns where more Miamians moved to a county than people from that county moved to Miami. Black lines represent population switches where Miami gained more residents than it lost. Yes, that's a whole lot of red.
Unsurprignly the black lines are concentrated in the North East, which probably represents the fact that Miami-Dade is still a prime destination for retiring New Yorkers and New Englanders. The county also sees an influx of residents from Michigan.
In most cases it seems that the resident Miami gains are usually richer than the ones it loses. For examples, in 2008, 250 people moved from Miami-Dade to Manhattan. They had an average income of $40,800. However, Miami gained 268 former Manhattanites who had an astounding average income of $84,500.
What's perhaps more surprising though is that Miamians are (in one of the few possible sentence in which this would make sense) moving north the South.
693 Miamians moves to Fulton County, Ga where Atlanta is located. In return only 325 ATLiens landed in Miami. Similar patterns are seen in surrounding Georgia counties, parts of North Carolina, and parts of Texas.
Miami also seems to lose more residents to the Pacific coast than it gains.
Of course, the map doesn't present the full reality of migration to and from Miami. The IRS only reports patterns when 10 or more residents move from one county to another. Obviously, the map also doesn't account for international movement.