I can only hope this is true, I am a big fan of Harley Quinn from Batman : TAS and I would love to see her in this with the Joker.
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http://www.aintitcool.com/node/33863

Hey folks, Harry here... According to ThisIsNottingham.co.uk there's a rumor that a British Soap star has traded in the suds, for pancake makeup and lap position on Heath Leadger's JOKER in THE DARK KNIGHT. Or is she? There's a lot places for a beautiful actress in a small part in THE DARK KNIGHT, but at this point, I've heard nothing about Harley Quinn - or even her pre-identity appearing in the film - and nothing in any of the original source articles give details on her role. So - as much as many of you want to see the Joker's wet dream, and ours... Nolan has been grounding this series more than making it a Where's Waldo affair. But who knows - maybe the toy companies have gotten to him...
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Another one about Sarah Michelle Gellar possibly playing her.

http://spaceramblings.blogsome.com/2007/03/29/sarah-michelle-gellar-to-play-harley-quinn-in-the-dark-knight/

Rumor has it going around that Sarah Michelle Gellar will be appearing as the Joker’s sidekick, Harley Quinn the Batman Begins sequel, The Dark Knight. Harley Quinn \ Dr. Quinzell, was a character created in the Batman animated series, which then appeared in the comic books and now will apparently make the transition to film as well.