An 80-page report from the Pew Forum (PDF) published October 9 contains some interesting statistics about the current state of declining religious belief in the United States. In the latest survey, 2.4 percent describe themselves as atheists, 3.3 as agnostics, and 13.9 as "nothing in particular."
For comparison, about 1.8 percent of Americans are Jewish, so assuming the sample numbers reasonably reflect the overall population, there are three times as many atheists and agnostics in the U.S. as Jews. (There is probably some overlap here, since many people who identify themselves as Jewish culturally are not religiously observant, including a fair number of unbelievers.)
The total fraction of those not identifying with a religion (meaning atheists, agnostics, and "nothing in particular" considered as a group) has increased from just under 15% to just under 20% in the past five years. Some of this may be statistical noise, but the sample size is large enough that the margin of error is narrow. This religiously unaffiliated are not entirely without religious attitudes, however. More then 2/3, for example, say that they do believe in God. More than a fifth of them even pray daily.
Another indication that beliefs are changing is the breakdown by age. Among those 18-29 years old, 32 percent are without religion. The percentage drops to 21 for the 30-49 age group, 15 for those 50-64, and just 9 for people 65 and up.
Surveys conducted by the National Opinion Research Center at the University of Chicago have shown very similar results. The religiously unaffiliated numbered under 10 percent of respondents in the General Social Surveys from the 1970s through the early 1990s but then began to rise, reaching 18 percent in 2010. (The corresponding figure from Pew that year was 17.4.)
The unaffiliated are relatively more likely to be male, unmarried, and young. Percentages are highest in western states and lowest in the South, with the Northeast and Midwest in the middle.