U.S. Census: Migration is expected to add one person every 30 seconds.
Via US Census Bureau News

(Emphasis added)

As our nation prepares to ring in the new year, the U.S. Census Bureau today projected the Jan. 1, 2008, population will be 303,146,284 -- up 2,842,103 or 0.9 percent from New Year’s Day 2007.

In January, the United States is expected to register one birth every eight seconds and one death every 11 seconds.

Meanwhile, net international migration is expected to add one person every 30 seconds. The result is an increase in the total U.S. population of one person every 13 seconds.