Rocksteady has announced that Catwoman will be a playable character in Batman: Arkham City, with her sections forming part of the main story.
She'll mirror the dark Knight with her own set of combat moves and navigation system, and you'll be able to upgrade the character as you progress. Her portion of the game will be very small compared to Batman, but still offer a interesting change of story, if not pace, to the world's greatest detective.
Expect trailer and impressions to be coming out of E3 soon.