Why Evolution Is True blog analyzes the data which shows religiosity of Americans has stayed relatively constant over time (although he makes a slightly different claim):
To me a difference of 81 (very or fairly important) to 19 (not very important) in 2011 is not that different from a 84% (very or fairly important) to 16%, though I guess 3 percentage point movement could be considered noteworthy, if it's not just a blip covered by the margin of error in the poll.
Why Evolution is True focuses on the analysis by Gallup of their data in which they claim that 40% of Americans are "very religious" while 32% of Americans are "nonreligious" and the remaining 28% are "moderately religious." (This last term seems oxymoronic to me, like being "a little bit pregnant.") The blog tries to figure out the discrepancy between the 32% who are nonreligious and the 1.5% who call themselves atheists or agnostic (i.e. like yours truly!) versus the 10% of Americans who say they don't believe in G-d.
Gallup summarizes their overall results with this analysis:
The imbrication of religious belief with political ideology in the country is a rather pernicious phenomenon, in my opinion.America remains a generally religious nation, with more than two-thirds of the nation's residents classified as very or moderately religious. These overall national averages, however, conceal dramatic regional differences in religiosity across the 50 states and the District of Columbia. Residents of Southern states are generally the most religious, underscoring the validity of the "Bible Belt" sobriquet often used to describe this region. Coupled with the Southern states in the high-religiosity category is Utah, the majority of whose residents are Mormon -- the most religious group in America today. On the other hand, residents of New England and a number of far Western states tend to be the least religious.Religion is related to politics in today's America, and it is clear from a glance at Gallup's State of the States map that the most religious states in the union generally are the most Republican, while the least religious states skew more toward the Democratic Party. This means that the most divided states -- and thus, those where most of the heavy-duty campaigning in this year's presidential election will be taking place -- are the ones where residents tend to be neither at the very religious nor at the nonreligious end of the spectrum.