The only rumor was that Dr. Harleen Quinzel would make a *cameo* appearance, but nothing substantial. I suppose she show up at the end of BB2 after the Joker's capture.
I'm not a fan of the character, myself. I think she's a two-bit henchwoman that the Joker is far better without. I wouldn't mind a cameo appearance by a Dr. Harleen Quinzel (perhaps, according to her origin, she could even set the Joker free in BB3), but otherwise, I don't want to see her. She doesn't even really seem to serve any good purpose as a character.
If I had my way, I'd play out a lot of her story. Have Joker assigned to her, have him destroy her mind, have her release him, and then have the Joker murder her after the escape. It would be a really terrifying move on the Joker's part.
But please, if they cast her, don't cast Brittany Murphy. She's incapable of giving a good, subtle performance - she's one of the worst working actresses around.