The cat has been let out of the bag. Ms. Selina Kyle, Catwoman, will be a playable character in Batman: Arkham City. But you already read that in the title.
The real news is the fact that this isn't just a re-skinned Batman with a slightly altered move set, or another character that's only playable in special modes (as was the case with the Joker in Arkham Asylum). Catwoman will have her own moves, actions, and gadgets to upgrade.
Since she lacks Batman's cape and grapple line, she'll rely on her trusty whip and cat-like agility to navigate the towering Arkham skyline. Playing on Ms. Kyle's thieving ways, instead of having Batman's Detective Mode, she'll feature a Thief Mode that helps her more easily see the info she needs to get her paws on.
That's just just scratching the surface of what Catwoman has to offer Batman: Arkham City when it releases later this year. We'll be sure to gather more information next week during E3. Stay tuned.