2005 United Van Lines Migration Study
United Van Lines has tracked state-by-state migration patterns since 1977. From its most recent report:

For 2005, the accounting is based on the 226,353 interstate household moves handled by United among the 48 contiguous states, as well as Washington, D.C. In its study, United classifies each state in one of three categories — “high inbound” (55% or more of moves going into a state); “high outbound” (55% or more of moves coming out of a state); or “balanced.” Although the majority of states were in the “balanced” category last year, several showed more substantial population shifts.

For the first time since 1995, California ranked as a “high outbound” state (55.7% of moves outbound).
United Van Lines Releases 2005 Migration Study [United Van Lines]