The Hollywood Reporter has learned that Eva Green has entered into negotiations for the part. Rumor has it that Artemisia is the central character of the story, even though it has been casting some male leads. Artemisia is the goddess who manipulates main character Xerxes (allegedly still being played by Rodrigo Santoro) into battle against the Spartans.
The other actor rumored to be involved in the film is Joel Edgerton, who is in talks to play Themistocles. He isn't locked down yet, though, so Green might be the first official actor in the movie if she ends up taking the role.