According to http://www.census.gov/ipc/www/idb/worldpopinfo.php
U.S. 310,204,105 11:34 UTC (EST+5) Sep 09, 2010
That's odd, because a 2008 estimate says 312 million. How is the US losing that many people in the past 2 years? 
- Emigration, death and birth control are the three main reasons to lose people. Estimates BTW are rarely right.