Clay Aiken glided into the Theater at Madison Square Garden on Thursday night to give voice to a Christmas wish that we can all relate to: a world without flash photography. "Gimme a camera and I'll flash it at you," Mr. Aiken purred, smiling wide so no one could mistake his offer for a petulant threat. This was a night when theater ushers, too often hidden behind a drab facade of jacketed professionalism, got a chance to perform backup vocals for the guy onstage. As Mr. Aiken crooned "Have Yourself a Merry Little Christmas," the ushers contributed a gruff basso...