The United States will enter 2012 with a population of roughly 312.8 million people (or exactly 312,780,968 people, if you want to be pedantic), according to the U.S. Census Bureau's end-of-2011 estimate.
That 312.8 figure is a 2.25 million person increase since New Year's Day, 2011. That's a 0.7 percent jump, which means we're growing at a slightly faster rate than entering 2011 (a paltry 0.6 percent) but not quite as fast as when we entered 2010 (when we boomed along at 0.9 percent).
As with the last two years, the Census estimates that there will be one birth every eight seconds and one death every 12 seconds. (So since you started reading this blog post, we've gained four and lost three.)
In addition, net immigration is expected to add one person to the population every 46 seconds. That's a small drop from last year when one entered every 45 seconds, but a relatively sharp drop from the start of 2009, when one entered every 36 seconds. That may seems like a small change, but consider it over the course of a full year (which has 31,556,926 seconds, according to the Google, which I assume is working off a regular 365-day year, not a leap year like 2012). At one new migrant every 46 seconds comes out to more than 686,000 people in a year, whereas one every 36 seconds comes out to nearly 877,000 people in a year.
Between births, deaths, and net immigration, the population is expected to tick up by another person every 17 seconds. That's a slight decline from the start of 2011 (one every 15 seconds) and the start of 2010 (one every 14 seconds).