Knowledge@Wharton cites research (PDF) from Peter Linneman and Albert Saiz about how population growth will happen in the US until 2020.

"The greatest future growth in the United States is likely to take place in the West, the Sunbelt and along the I-85 corridor between Raleigh, N.C., and Atlanta, Ga. In a literal sense, Americans are following the sun, since factors such as the number of "bright" or "sun" days in January and the absence of winter heating costs are significant aspects of this anticipated redistribution of population.

Rapid growth will be concentrated in warm, sunny locations like California, Arizona, New Mexico, Florida and the greater Seattle metro area. Other places where new arrivals will add to the overall population in a significant way are Salt Lake City, the Denver North-South corridor, the state of Texas, the Atlanta-Charlotte-Raleigh corridor, and the Chicago-Madison region. What the authors term "substantial population inflows" can be projected for Atlanta, Chicago, Phoenix, New York, Dallas, Houston, Los Angeles, Orlando and Denver.

By 2020, the total U.S. population is expected to reach 336 million people. That factors in an anticipated increase of 53.7 million new Americans between 2000 and 2020. (The U.S. population is projected to reach 300 million this fall, according to a report from the U.S. Census Bureau earlier this week.) "

America has very favorable macro-economic data in population growth and productivity growth. Even after the real estate boom and bust (to some extent at least now) it seems that investing into residentail real estate in the counties emntioned in the report can be very much worth the while.