Saw this about 2 weeks ago, loved it, and wanted to write a whole post about it:
Now, been sitting on it too long, don't remember what I originally wanted to say.
It's probably important to note that this seems to measure plurality, not majority, so even in the heart of Georgia it's possible that most people are not Southern Baptist, they're just too divided among other groups to swing the needle.
It also doesn't indicate how many people are religious in an area; there's no measure of intensity. That means you could have areas where every religious person in that county belongs to X, and yet they are still only 1% of the population. In other words, we're not a predominantly Catolic nation, as this map appears; they just have the most consistent geographic distribution. Compare to this map, showing density of religious adherants, across all religions:
The color scheme probably helps, but I was also reminded of the electoral maps from recent presidential elections. Compare to this one showing shift in voting patterns from 2004 to 2008:
Raises some questions: are Catholics more liberal than we thought? Are they attracted to liberal areas? Does a concentration of Baptists increase conservative tendencies? Combining this with the earlier observation, is the reason some Catholics feel persecuted that they are the largest religious group in ares of the country where not many people are religious?
I also find it interesting that all the non-Christian religions get grouped under "Other". That suggests none of them had more than the LDS's 81 counties (not surprising), and I'm sure some of those "Others" are also Christian varietals, but still.
Talk about being made the outsider; they literally labeled all Jews, Muslims, Hindus, Buddhists, and every other Asian religion as The Other, a term usually applied to groups to explain why they are hated and/or feared. Heck, looking at this map you'd think there are no non-Christians in the country! That disturbs me more than a little.