Warner Bros. is hoping to get Justice League out into theaters as soon as possible, following the success of Disney's Avengers. Christian Bale is done with Batman, though, so WB will have to resort to the next best thing.
Spoilers ahead, if you haven't seen Dark Knight Rises (in fact, this entire article could be considered a spoiler, but still). At the end of Rises, Joseph Gordon-Levitt's character is given full access to the Bat-Cave after Bruce Wayne retires from the job. So for the time being, Levitt is WB's Batman.
However, there isn't time for there to be a Batman film before Justice League, so our first glimpse of Levitt could come in the next Superman film. HitFix reports that Zack Snyder is making a few changes to his Man of Steel movie so that it would fit into a Justice League continuity, just as Disney's Thor and Captain American tied into The Avengers. A source told the site, Joseph Gordon-Levitt absolutely will be appearing in Justice League as the new Batman.
Will JGL appear in Metropolis to ask for Superman's help? It certainly would make for a wild post-credits cameo.