The good news: There's a very cool Sinestro Corporation* Batman: Arkham City skin you can get that will look smashing. The bad news: You have to buy a limited edition of The Green Lantern on Blu-Ray until the skin is released as DLC. Click this link to take a look at the costume.
Joystiq's super-secret sources say the skin will be available to purchasers who buy The Green Lantern blu-ray at Best Buy and other as yet-unknown retailers.
*I'm not going to get into a long discussion about the ridiculousness of Batman wearing a Sinestro uniform, but, believe me, I could write thousands and thousands of words about it. I will, however, point out that The Green Lantern was a weak-ass movie.