Sienna Miller has signed up to star in a movie version of GI Joe. The actress will play Baroness, a sexy femme fatale skilled in espionage, reports Variety. The story is about an international force of operatives who use high-tech equipment to battle Cobra, an evil group headed by a Scottish arms dealer. Stephen Sommers directs the live-action film based on the Hasbro toys, comic book and TV series. Shooting is scheduled to start in February 2008 for a planned Aug. 7, 2009 release. Miller, 25, is the first star to sign on for the big-budget production.