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.
Mormons account for 1.7% of the population, Muslims 0.6%, and Hindus 0.4%.
Those 18-29 were more likely to be unaffiliated with any religion than those over 70.
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.