Robert Downey Jr. is officially back in the superhero business.Production started Wednesday on "Iron Man 2," the sequel to his 2008 blockbuster about the Marvel Comics character.Downey reprises his role as a billionaire genius who fights bad guys in a high-tech metal suit that’s loaded with gadgets. "Iron Man 2" reunites Downey with co-star Gwyneth Paltrow and also features Don Cheadle, who replaces Terrence Howard as the superhero’s military ally.
