Well, religion and intelligence have been shown to be inversely correlated. Perhaps 'intelligence' is the wrong word; educational level might be better.
For example, only 7 percent of the members in the U.S. National Academy of Sciences believe in a personal god [1]. I forget the numbers for the general public of the US (I don't have the book with me), but it was the vast majority who believed in a personal god.
Of course: grains of salt, correlation vs. causation, etc.
I don't think being an atheist requires a certain level of intelligence, but I do think that people of a certain level of education have been taught to think critically about everything they read/learn/work with, which probably bleeds over into their personal life.
[1] Victor J. Stenger. Has Science Found God? p.78