This WaPo story describes this Baylor University study (long pdf alert). The authors include two very prominent sociologists of religion,they acknowledge other big guns in the field, and the Gallup Organization conducted the survey, so it has a good bit of professional credibility. In addition, the authors promise that this is merely the first of many future surveys. The Pew Forum/Pew Center folks have some major, high-powered competition.
There’s all sorts of interesting demographic data, such as these tidbits: a significantly higher proportion of females than males identify with black Protestant churches, while a higher proportion of men than women identify with Roman Catholicism; and young people (18-30) are most likely to declare no religious affiliation, largely at the expense of the Catholic church, which seems significantly underrepresented in that cohort (10.1%, as opposed to 20% or more in other cohorts).
The study also found that the attributes most closely connected with political conservatism are relative biblical literalism and church attendance: it turns out that relatively "biblically literalist" Catholics, evangelicals, and mainline Protestants are more or less indistinguishable in their political views.
There are all sorts of other interesting findings in the survey, but you’re going to have to see them for yourself.