Arguably the most nonreligious region in the world, Western Europe has been known for its large population of the self-described "none" when it comes to religion. The 2005 Eurobarometer Poll puts the number of atheists in the EU at 18% and the borderline spiritualists at 27% while only 52% said they believed in a god. According to a study done by USA Today back in early 2009 that compares religiosity in America at the present and in 1990, all religious denominations have seen a decline in numbers. Furthermore, the "no religion" number has nearly doubled, shooting from 8% in 1990 to 15% here in 2009. I attribute these numbers mainly to generational differences such as social liberalization, a repudiation of "traditional" values, easy access to scientific information, and a general disillusionment with religion itself. Nearly all of these reasons were also factors in Europe's own religious decline along with the centuries of Christian/Muslim warfare and the Enlightenment period.
I predict that with the passing of future generations, religiosity will continue to decline until we reach a period of stagnation as churches attempt to completely entrench and focus their efforts in the South and Midwest. I believe that, in looking at the rates of decline in other regions of the world, the non-religious number (this is including atheists, agnostics, and some borderline spiritualists) in the US may hit a peak of 35% (perhaps as high as 40%) before the stagnation based on applying the same trends to the US as in Europe but with slower rates and eventual flat-lining. But this probably won't occur for at least another 50 years (assuming the number of non-religious grows by 4% each decade). It is unclear whether the non-religious could ever get to a majority in the US, but if so, it would take a long time. The time in which the non-religious gain a true majority in Western Europe is very close. And the very sight of a majority non-religious population in the world will be a marvel, unheard of in human history. The only question may be if, or even when, the US follows suit.