Website Bleeding Cool claims to have heard "from multiple sources" that an animated movie based on the classic Frank Miller miniseries The Dark Knight Returns is in the works. The feature would supposedly be in the same format as other DC projects such as the recent All Star Superman -- which would mean, I would guess, that the style of the animation would match that of Miller's original work.
This is purely speculation at this point, though admittedly it is rather fun speculation. All-Star Superman was a pretty faithful adaptation of Grant Morrison and Frank Quitely's epic work, and a comparable cartoon based on Dark Knight would be pretty cool. Until -- or if -- we find out any more solid info on this project, we'll have the already-in-the-works animated adaptation of Batman: Year One -- in which Eliza Dushku is rumored to voice Catwoman -- to tide us over!