This rumor pretty much had to happen, right?
As I'll assume you'll recall if you saw or at least by now read the Wikipedia summary of The Dark Knight Rises, the end of the film--unless interpreted as elaborate hallucinatory defensive mechanism to keep Michael Caine sane--implies a future in which Joseph Gordon-Levitt's (Robin) John Blake could take on Batman's rubbery mantle, perhaps going on to fight such unexplored villains as Philip Seymour Penguin, or LadyRiddler. But with director Christopher Nolan departing the series, and both he and Warner Bros. suggesting they had closed the book on Nolan's world, it seemed it was a future meant only to be suggested but not explored, no matter how ridiculously profitable that would be.
Except that now it seems Warner Bros. may indeed wish to explore the matter, perhaps because of the profitability thing, as Hitfix claims that apparently-reputable sources have told them "Joseph Gordon-Levitt absolutely will be appearing in Justice League as the new Batman." They go on to say that we won't even have to wait until Justice League to see just how the Batsuit hangs off Gordon-Levitt's slight frame: remember how Zack Snyder subtly implied his Superman film could tease at the formation of the Justice League? Well, supposedly that could mean an appearance by Joseph Gordon-Batman, as well "at least one other actor from the Nolan films." Could The Man of Steel close with a little meet-cute between Superman and our new Batman? And will Dark Knight reporter Anthony Michael Hall see it all, look directly into the camera and mouth, "What a scoop"? Yes.