Threatening letters critical of the interracial marriage between a Tony Award-winning white actress in the Broadway smash "Wicked" and Taye Diggs, a popular black film and TV actor, have sparked a NYPD probe and an increase in security at the famed Gershwin Theatre, sources said. One of the letters "threatens bodily harm to Taye Diggs," according to a police source. That letter was received Wednesday by Jed Bernstein, president of the League of American Theaters and Producers, the industry's major trade group, with offices on West 47th Street. Sources said that before Wednesday, two other handwritten letters were sent to...