From the U.S. Census Bureau:
As our nation prepares to ring in the new year, the U.S. Census Bureau today projected the Jan. 1, 2012, total United States population will be 312,780,968. This would represent an increase of 2,250,129, or 0.7 percent, from New Year’s Day 2011, and an increase of 4,035,430, or 1.3 percent, since Census Day (April 1, 2010).
In January 2012, one birth is expected to occur every eight seconds in the United States and one death every 12 seconds.
Meanwhile, net international migration is expected to add one person to the U.S. population every 46 seconds in January 2012. The combination of births, deaths and net international migration results in an increase in the total U.S. population of one person every 17 seconds.