Jacob Sullum summarizes new research on the subject:
[A] recent study led by J. Paul Leigh, a professor of public health sciences at U.C.-Davis, finds that poor people eat in fast food restaurants less often than the middle class. Looking at data from the Continuing Survey of Food Intakes by Individuals and the Diet and Health Knowledge Survey, Leigh and co-author DaeHwan Kim found an inverse relationship between income and fast food consumption up to an annual household income of $60,000. Beyond that point, people were less likely to eat fast food.
2009 map of every McDonald's in America from here.
The Dish 