People on the move...middle class moving from expensive cities to cheaper cities

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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.

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I bet if you looked at population density per square mile growth then SF, LA and NYC would lead. 

I would also guess that the growth in population from high to lo consisted of non home owners in the high areas.  There are equity immigrants but I think their numbers are generally overstated.