It was officially announced and relayed by IGN at Comic Com that an animated version of The Dark Knight Returns is in fact in the works and planned for release in 2012, possibly to coincide with the DVD release of The Dark Knight Rises. The most surprising element of this report is that, following in the footsteps of Harry Patter 7 Parts 1 and 2, the movie will be split into 2-parts.
As for the writing? We were promised a very faithful version. Does that mean R-rated? I would have to think no again for financial reasons, but its possible they may really push that PG-13 boundary... WHICH THEY SHOULD. The whole point of these DVDs was to target grown up nerds like me. Yes, I also watch Batman: The Brave and the Bold, but these DVDs were intended enable more mature themes and content that was not possible on Saturday morning (or Friday night, or whenever). I don't mind the occasional lighter tale as in the superb All Star Superman, but to date, my favorite movie was the very dark Batman: Under the Red Hood.
Since this is scheduled to be the third (and fourth?) release in 2012 following Justice League: Doom and Superman vs The Elite, it may be some time before more details arrive, But until then, we still have the adaptation of the brilliant Year One to anticipate.