The first official 2010 Census results are out today. As of April 1st there were 308,745,538 residents of the United States. California, the most populous, had 37,253,956; Wyoming, the least, had 563,626.
We have a decennial census in the U.S. because our Constitution mandates it. Every 10 years, we reapportion representation. This time, very much like the last time, Illinois, Missouri, Michigan, Pennsylvania, New Jersey, and Massachusetts are losing a seat; New York and Ohio are losing two; Washington, Nevada, Utah, Arizona, Georgia, and South Carolina gain one; Florida gains two; and in a sign of the Apocalypse, Texas gains four. Louisiana also lost a seat, most likely as a result of people fleeing after Katrina in 2005 (though the state did have a net gain of around 70,000 people). With these results, each member of the House represents about 711,000 people.
Oddly, only Michigan and Puerto Rico lost population since 2000. Nevada had the biggest proportional gain, its population increasing 35%. Texas had the largest numeric gain, of about 4.5 million. Other big gains include North Carolina (about 1½ m), Arizona (25%), Utah (23%), and Idaho (21%).
The Census has an interactive tool that has data back to 1910 for more information.