In a piece passed wildly around the net, Batman on Film is reporting that Nolan is very uninterested in returning to Gotham City. The story goes that Heath Ledger's death upset him greatly, and dismantled any and all plans he had for a third installment. The Joker was set to return (is that really a surprise?), and now there's simply no story. While Nolan and David S. Goyer are working on ideas, it's unlikely you'll see anything before 2012 at the earliest, with 2013 being more likely.
Considering Nolan is about to start shooting Inception, it's very possible that he's doing the right thing, and staying focused on one massive project before flinging himself into another Batman. But he has been diffident about committing, and about discussing story ideas, which is odd considering the way The Dark Knight ended. Vacations and fresh projects are neccessary to anyone, but it does seem peculilar that he isn't more enthusiastic about returning to what he left off. Contrast him with Jon Favreau who hasn't broken a sweat over Iron Man 2.
But Favreau might actually be a little clue as to what else could be going on. The astute Devin Faraci suggests that the rumors could simply be a ploy to reopen contract negotiations in the press, and get a little more money, which is something Favreau did, and quite successfully, too. Whatever the story, Warner Bros does have a possible list of replacements ready for the asking.