A lot of media attention (e.g. - here) has focused on the new American Religious Identification Survey of American's views on religion, which finds that 15% of Americans now say they have no religious affiliation, up from 8% in 1990.
Lack of religious affiliation doesn't necessarily imply atheism however. When asked whether they believe in God, only 2.3% of ARIS respondents said that "there is no such thing" as God. However, 5.7% said that they are "not sure," and 4.3% said that "there is no way to be sure." These two latter answers might be categorized as agnostic. Unfortunately, ARIS didn't ask this question in 1990, so we do not know whether the proportion of atheists and agnostics has increased since then.
The ARIS survey may underestimate the true prevalence of atheism. Because of widespread societal prejudice against atheists, some survey respondents might be hesitant to admit their atheism, even in an anonymous poll. We know from polls on other issues that survey respondents often hide their true beliefs when these conflict with perceived societal norms. I suspect that at least some of the people who gave agnostic responses are actually atheists.
The same may be true of some of the 12.1% who picked the answer stating that "There is a higher power, but no personal God." Ironically, this vague phrasing might be perfectly compatible with atheism, depending on how it is interpreted. Assuming that the "higher power" you believe in is not omnipotent, omniscient, or completely benevolent (the standard attributes of God as depicted by the major monotheistic religions), even the most convinced atheist could potentially choose this answer. For example, I consider myself an atheist in the sense that I believe that God as defined above almost certainly does not exist. However, I also think it's perfectly possible that there are extraterrestrial "higher powers" who are vastly more powerful than we are. UFO enthusiasts notwithstanding, I certainly don't believe that the existence of such superpowerful ETs has actually been demonstrated. But neither has anyone definitively proven that they don't exist.
UPDATE: Apparently, ARIS actually did give respondents the opportunity to identify themselves as "atheist" or "agnostic" on one of the other questions in the survey. Only 0.7% of respondents picked "atheist" and 0.9% chose "agnostic" (both numbers up slightly from 2001).
This result powerfully illuminates the social stigma attached to identifying as an atheist. More than two-thirds of the 2.3% of respondents who said that "there is no such thing" as God still didn't self-identify as atheist even though that is what they clearly are. Some of these people may simply be confused about the definition of the word "atheist;" but I doubt that is the main reason for the discrepancy between the two questions. Equally interesting, some 10% of respondents gave answers indicating that they are unsure about whether God exists or not, yet only 0.9% call themselves "agnostic." Here, respondent confusion may play a bigger role, since the term "agnostic" is probably less widely known than "atheist."