New study just released from the Pew Forum on Religion & Public Life.
They found that 44% of the US adult populace does not belong to the faith they were raised in.
"For example, roughly two-thirds of those who were raised Catholic or Protestant but now say they are not affiliated with any particular religion have changed faiths at least twice in their life, including those who have changed within the unaffiliated tradition (e.g., from atheist to agnostic)."