Well that wasn't so bad: after a relatively short delay, Batman: Arkham City - Harley's Revenge has come to the PC for $10. It includes a single-player mission playing as Robin to find Batman after he's captured by a grieving Harley Quinn.
As a hopeless Batman combat addict, I'll definitely play this when I get back from E3, even though it means suffering through a layer of Games for Windows Live crap to do it. It's reviewed fairly well elsewhere, with the main objections being that it's on the short side and doesn't introduce enough new gameplay. But hey, more Batman is more Batman, so I'm on board. Are you, or have you moved on to the next big thing?