Hmm, I've always been curious how the atheist-group activity looks in cities like Boston. I would expect a lot of activity in cities like Austin, Texas, for example. We've got a bunch of active groups here, too.
Liberal cities surrounded by a sea of Red would have both the clustered population to get a group together and the feelings of persecution, on a personal level. Rural areas would have the feelings of persecution but not the clustered population, making it difficult to any reasonably-sized group together. Liberal cities in heavy-blue states have the clustered population, but I imagine most of the local atheists are pretty slack, since they're not swimming in a sea of constant religiosity.