I'll admit, it was quite a jarring bit of news. So many Batman fans grew up with Ms. Sorkin's Harley. Because she voiced her from day one of her creation, she was (and apparently still is to many) Harley Quinn. Though I shared the concern, I went back and listened to the Trailer, as well as the few audio snipets you can find of Arkham City Harley dialogue, and, frankly, Tara's taken that ball and run with it in a big bad, and good, way. There is a very subtle difference, as those who've grown up with the Arlene-Harley will hear, but Tara really keeps the character close to home. It's not like, say, the interpretation in The Batman. Ugh.
Also, this really comes as no surprise. In Batman: TAS, Harley was portrayed as the crazy, tongue 'n cheek bumbling Hench-girl. Over time, they tried to lay on the risque humor, "Come on, Puddin. Don't you wanna Rev up your Harley?" But Arlene's portrayal of the character was so slap stick, it didn't really work. Even in Arkham Asylum, despite a great performance, one could tell she was kind of struggling with the new direction they were taking the character in, i.e. taking her from Comic Relief to full on "Shag the crap out of her" Sex-Pot. As we've seen from the Trailer and screenshots, they're taking that route even further. And Tara, despite a lot of the Gumdrop and Lolly-pop roles she's played, the most recent being the *Awesome* Twilight Sparkle in MLP, can go super sexy + weird if the character calls for it.
And I think, at the core, that's why Arlene passed this time. Maybe it was her age, or maybe just a dislike of where the character is going, but I just don't think she had it in her to be "Do me, Batman," Harley as opposed to her earlier incarnation. For those who can't imagine Harley without Arlene, she's still performing and recording dialogue for the character in DC Universe Online, which has the classic portrayal of the character. At least, for now.