Although it is an arbitrary benchmark, the United States is nearing a population of 300 million, and is expected to surpass this number sometime this month. You can watch the U.S. Census' very own population clock here.
In just one more way, the United States does not resemble other industrialized and developed nations, such as much of Europe, Japan, and even China, in that it has a growing and increasingly youthful population, largely due to immigration. As William H. Frey writes in the LA Times,
"Odds are that the 300-millionth American will be the child of immigrants or an immigrant himselfas likely as not born or residing in Los Angeles. Right now, about half of U.S. population growth in any given year or month is attributable (through migration or natural increase) to Latinos, the vast majority of whom are either first- or second-generation Americans."
The Christian Science Monitor (via AlterNet) has an article on the environmental impacts of the U.S. population. Reuters (via Scientific American) also has a brief analysis of the environmental impacts. The Boston Globe also has an article on the wave of planning for this growth here. And the Brookings Institution, a Washington DC-based think tank, has an extensive page of links to their own demographic and political research.
With the huge role the United States plays in the world, geopolitically, culturally and environmentally, it's vital to understand long-terms shifts in the character of the country, and demography is a critical driving force.