People on the move...middle class moving from expensive cities to cheaper cities
Here's an interactive map that uses the 2000 and 2010 census data.
http://projects.nytimes.com/census/2010/map?ref=us
Notice the following:
Larger, expensive cities have not seen large percentage increases in population (ex. San Francisco just 3.7%, Los Angeles 3.1%, NYC about 4.5%)
Cheaper expanding cities experienced significant population growth (Austin 26.1%, Dallas Area 30-50%, Houston 20.3%, Raleigh 43.5%)
The Midwest, particularly South Dakota, North Dakota (except Fargo), Nebraska, and Kansas, Iowa, and Montana experienced little growth
I would expect that in the last four years we've seen these trends continue...people moving, primarily the middle class, out of the more expense cities to more affordable cities.