For eight years, the president of the United States — the ex-officio socialite in chief — has turned up his nose at the slightest whiff of social aspiration. Now a new generation of Washington socialites has a dream. They see a dashing, young, brilliant president and his equally brilliant wife ushering in an era of inclusive entertaining. Gone is the white-gloved, debutante-loving, Pilgrim-pedigreed culture that still mattered in Kennedy’s Camelot and Reagan’s Hollywood East. Starting Jan. 20, when President Barack Obama and Michelle move into the White House, the “in crowd” will include just about everyone who, since the dawn...