It was only a short time ago that Marvel Executives said that R-rated films weren't going to be a part of their strategy going forward, and it seems like Warner Brothers and DC comics are following suit. According to a source inside the studio known to our friends at IESB.net, Warner Brothers has also sworn off doing R-rated tentpole films.
This decision most likely is one of economics. Iron Man and The Dark Knight both had incredibly good box-office takes, as have other PG-13 superhero films like Batman Begins, Fox's X-Men trilogy, and Sony's Spider-Man films. Meanwhile, R-rated superhero films like Punisher: War Zone and Watchmen have struggled to reach profitable box-office takes.
What does this mean going forward? Well, it means that future superhero projects out of Warner Brothers will probably remain in the PG-13 realm, meaning that families will go see the films together, kids will buy the toys of the superheroes, and the studio will make money. So, expect Green Lantern, The Flash and (if it ever materializes) Wonder Woman all to end up rated PG-13.
Now, this doesn't completely preclude the possibility of R-rated films that are based on comics. The studio says that Jonah Hex will probably still be rated R. However, the studio doesn't consider that a superhero film - just one that is based on a comic. So, other movies based on the Vertigo comics imprint from DC Comics might still be adapted into R-rated films. But it does mean that we probably shouldn't expect to see an R-rated The Dark Knight Returns coming anytime soon.