PZ's readers are in a tizzy over this somewhat counterintuitive map:
Notice something weird? If the "Bible Belt" is measured by "religious adherents," then it is slapped vertically across the middle of the country, not in the south. Something is wrong here.
Religion means many things to many people. There are people who have a personal relationship with Jesus Christ and who try to convert people to this state who don't go to a specific church on Sunday but meet with a roving "fellowship," reject "religion" and sometimes even the term "Christian." There are Unitarian Universalists I know who go to church every Sunday, but express the same non-theistic naturalistic understanding of the universe I do. These are the nuances missed by this map, because what it is tracking is church affiliation. You can see many more maps of this sort here and note the geographic variation in denominations. This matters, because different denominations differ in how they measure church affiliation. Groups on the evangelical Protestant wing of Christianity tend to demand a lot of participation and regular attendence to be a "member" of the church. In contrast, Roman Catholics tend to be more relaxed, especially after not attending mass was no longer listed as a mortal sin after the 1960s. For some Jews being a member of a temple or synagogue might mean paying for seats only during the High Holy Days.
There are also differences within Protestants between the more liturgical, from traditional denominations and the freeform non-denominational Christianity common in much of the South and West. In the former case a higher percentage of the population might be regular churchgoers because it is a conformist act which signals more than just belief in a set of religious propositions, rather, it tells others that you are part of the community, that you identify with the values of your folk. Operationally monoreligious Roman Catholic, Lutheran or Mormon towns might be like this. In contrast, in other regions of the nation where evangelical and non-traditional Protestantism is stronger, more flux might be common because religion is a more personal affair, and it is understood that one might have to stray before one can be redeemed (a milder version of Amish rumpsringa).
Here in the Pacific Northwest I think some of these issues come to light. On the one hand, we are rather non-churched, and out & out non-religious, but there are also many evangelicals amongst us. David Klinghoffer commented on this when he noted that though he knew statistically far fewer citizens of Seattle were churched than in New York, it seemed he encountered evangelicals far more often than back east. Here in the Pacific Northwest many who might have been "cultural" Catholics and Jews, who attended on Christmas or the High Holy Days, might be enjoying the great outdoors since there isn't a communal pressure to attend because "ethnic enclaves" here might mean the Norwegians in Ballard, WA. In contrast, those who do retain religious affiliation might feel more driven to be active because they can't be complacent in a sea of heathens.
Update: This map page is the best one.
You can see many more maps of this sort here and note the geographic variation in denominations.
Aw, don't use that link. This meme spread all around from that guy, and he made those small annoying graphics. But the real, original, maps have been around forever here.
Ok, I see he finally recently 'updated' with a link to the originals.
I think the common theme in that vertical belt is that those places are all very sparsely populated. The southeast still has a very significant proportion of its area in the 50-75 percent range, and the pattern is maintained even in more densely populated regions of the south.
Believe me, we've got plenty of Bible-thumping down here.
Bible thumping? Down here? In the South? Oh, yeah, bible thumping we got plenty of. One of my coworkers here in Alabama, a computer science guy, recommended answers in genesis for someone who was worried about how to reconcile evolution with her world view. I think there is a fairly strong streak of thumpism from Texas heading north. If any of you have driven along I-40 through Texas, you have probably seen what I call the Cross of the Mobile Homes. It's a gigantic cross made of what looks like the metal siding used on older mobile homes. It's really ... uh, something.
I love going through these maps and puzzling over all the myriad little anomalies. ("Why are there so many Mormons there??", etc.)
For instance I didn't know Alaska had such a big Orthodox population. Even more interesting when I checked and found they have hardly any Russians. Just the legacy of Russian missionaries.
The lack of adherents in the southeast is due to the presence of small denominations that do not report statistics, particularly African-American.