Dream city, Sin City, a mirage in the desert, Las Vegas is a film set in its own right, a piece of pop art, an outdoor museum of American culture. What is the story behind the neon lights and fantastical buildings? What will its future be in these tough times? Alan Yentob takes a mob tour and talks to producers and performers about the golden days when Sinatra and Dino held the stage, and the wise guys called the shots.