I think this phenomenon happens all over. Cities -- and regions within cities -- can have their own peculiar culture, while the culture of rural areas varies on much longer distance scales.

Because of this, I suggest that the idea that North American is best thought of in terms of regions might be an unhelpful one.

OTOH, the evidence from Facebook apparently suggests otherwise:

http://petewarden.com/2010/02/06/how-to-split-up-the-us/