This interactive site from the Pew Research Center explores migration flows in the U.S. on state and regional levels. To tie this into the planning realm, I’d guess there is at least some interplay between these flows and different types of land use/real estate development regulation among the states. (Of course, that'd be just one factor out of many). Beyond planning, it will also be worth watching how state-to-state policy differences in dealing with the general economic and budget difficulties affects these flows in the years ahead.
Link
Via Pew Social Trends