While the announcement of Batman: Arkham Knight yesterday didn't bring forth a release date, Gamestop seems to have one. In their pre-order page, they list October 14th as the game's release date, which seems pretty reasonable to me considering the history of the franchise's releases.
Also, if you pre-order the game, you'll get to play as Harley Quinn in four challenge maps. The look of Harley has changed quite a bit from the first game, something that I'm still getting used to.
Oh, you'll also get a nice little Batman poster with some comic covers celebrating 75 years of Batman.