Oh hey, doing a bit more research, it'll be out on May 30th!
http://www.xbox360achievements.org/news/news-11508-Batman--Arkham-City--Harley-Quinn-s-Revenge--DLC-Dated-and-Priced-%5BUpdated%5D.html
And apparently you get to play as Batman and Robin to defeat Harley and shut down Arkham City forever. It'll be a new storyline and everything. I like it!
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Xbox GT: PrivateBiscuit1
Down with Zhang. Let there be Biscuit!