What do Barack Obama, Samuel Beckett, Fidel Castro, Joan Baez, Conan O'Brien, Natalie Portman, and Dwight D. Eisenhower have in common? Their footsteps have all crossed paths in Harvard Square. This well-trod patch of Cambridge turf at the corner of the United States' most renowned university has long been a crossroads where poetry, retailing, politics, design, performance, and every other cultural endeavor intersect. From the square's tweedy aspect...