Who moves where

I found this  interactive map from Forbes Magazine at Kevin Drum's blog, who found it at Andrew Sullivan's. You click on your county, and it shows black lines indicating immigration and red lines indicating emigration. This is the map for LA. People move to San Francisco and Seattle but never the other way, but it's a magnet for the North-East. Who wouldn't rather live in LA than Detroit? It only shows moves of 10 people or more.