I've seen many shows on Broadway, and also here in Washington at National Theatre, Ford Theatre, and the Kennedy Center. I'd say the common dress these days is casual, although in the evenings you may see people very dressed up. I tend to wear clothes that I might wear to church or to a work function - dressier slacks and a blouse, and dressier shoes. I do it more for me, when I go out on the town, I like to dress up. It seems like its anything goes at the theatre, although I don't know that I have ever seen anyone wear shorts or T-shirts, I have certainly seen plenty of people in jeans and sneakers.