The US Census Bureau recently released its population estimates as of July 1, 2007. The data showing population change between 2006 and 2007 can be found here. The analysis showing components of population change can be found here. Notably, the population increase of just under 2.9 million amounted to only a 1% increase. International migration contributed only a little bit more than 1 million new residents or a measly .3% increase. A change of this magnitude is likely to cause minimal strain on the country's economic or social systems. Specifically, if one assumes a correlation between population growth and GDP growth(not unreasonable given that consumer spending makes up something like 70% of GDP), certainly 1% is nothing to get excited about in terms of GDP growth.
In terms of internal migration, the data shows that the Western region of the country had essentially zero net domestic migration. This is quite remarkable as for the last 30 years the West was attracting large numbers of migrants from the eastern part of the country.
The data distribution of population throughout the US is thought provoking. The government divides the country into four regions: Northeast, Midwest, South, and West. This categorization seems reasonable. The regions rank in population as follows: South-110 million, West-70 million, Midwest-66 million, and Northeast-55 million. So the region which has been the political and industrial leader of the US for most of its history now has the least population. Although there has been much discussion of outmigration from the Northeast and Midwest over the years, the magnitude of the gap in population levels between the traditional power centers and the South and West is now rather eye-popping. We should expect significant changes in US political and economic decision making in the future as a result.