A survey by the Pew Forum on Religion and Public Life of 35,000 Americans found that 44% of them have changed affiliations between “major faith traditions,” such as from Baptist to Methodist. 28% have switched between traditions.
Protestants make up a bare majority of Americans–51.3%.
16% classify themselves as not affiliated with any faith (despite only 7% being brought up that way), and 4% as atheist or agnostic.
Those 18-29 were more likely to be unaffiliated with any religion than those over 70.
Catholicism has had the greatest loss of adherents, offset by Catholic immigrants, while the unaffiliated have had the greatest gain.
UPDATE (March 8, 2008): A Los Angeles Times op-ed by mathematician John Allen Poulos, author of the recent book Irreligion, argues that the number of unbelievers is probably underreported due to the general public’s mistrust of atheists.