There was a film in the works a few years ago that was cancelled. After that the Green Lantern, Man of Steel and the sequel to The Dark Knight, plus another solo movie was going to set up Justice league. But then Green Lantern bombed and The Dark Knight Rises threw Bruce Wayne out the window. You could always set Justice League in that silly eight year gap where Batman does nothing but it seems unlikely.
Nolan is involved in both Batman and Superman movies and he has said he wants to keep them individual. At the moment they seem to be rushing into it so they can keep up with The Avengers.
And I agree, the Justice League cartoon was brilliant, as was the whole DC animated universe that began with the Batman animated series.
