Update (Mar. 15): A new report from the Institute for the Study of Societal Issues at the University of California, Berkeley, states that 1 in 5 U.S. adults reported no religious preference in the 2012 General Social Survey.
And although the percentage of respondents who specified a religious preference is declining, data suggests that belief in God actually is not declining. The report states:
[Respondents'] certainty of believing in God decreased more between 1965 and 1991 than since, while preference for no religion barely changed from 1965 to 1990, then almost tripled since 1991. This asymmetrical timing of changes indicates that the connection between faith in God and identifying with an organized faith, if there is one, is far from simple. Unchurched believers still far outnumbered completely secular people in 2012.
Update (Mar. 8): Bradley Wright has posted a new breakdown of the rise of the religiously unaffiliated over time based on age. Wright's newest analysis note that "the ...1