You know, I used to have this impression (it was something my mother did because of the subway makes her nervous and is inconvenient because they live 8 blocks form it -- which is hard to do in Manhattan), that it was something wealthier, older, white people did. And then I moved up to Spanish Harlem, which is also far east enough that the bus is closer than the train, and now I see the bus as not having class divides. I think it depends on the bus line, certainly, based on where it serves.