The new report from the Pew Forum is worth a look.
Among the findings:
The biggest gains due to change in religious affiliation have been among those who say they are not affiliated with any particular faith. Overall, the 2007 “U.S. Religious Landscape Survey” found that 16% of the adult population is unaffiliated, with the vast majority of this group (79%) reporting that they were raised in a religion as children.
Among the currently unaffiliated, large majorities of both former Catholics and former Protestants report attending worship services at least once a week as children (74% and 64%, respectively). However, regular church attendance drops dramatically by adolescence for both groups, and very few unaffiliated people report regularly attending worship services now, as adults. Unaffiliated former Catholics and former Protestants are equally unlikely to say they regularly attend worship services as adults.
Three-in-four former Catholics (75%) and former Protestants (76%) who have become unaffiliated say that many religions are partly true but no religion is completely true. Most of those who agree with this statement say this is an important reason they became unaffiliated, including 48% of former Catholics and 43% of former Protestants. About seven-in-ten (73% of former Catholics and 71% of former Protestants) say that religious organizations focus too much on rules and not enough on spirituality, with nearly half saying this is an important reason they became unaffiliated. A slightly smaller fraction of those who have become unaffiliated say that religious leaders are more concerned with money and power than they are with truth and spirituality, and about four-in-ten say this is an important reason they decided to become unaffiliated.