A rumor is spreading online that Russell Crowe will make a cameo appearance in The Dark Knight, but it's not true. The rumor comes from one site's (we won't mention who) failure to recognize that Christian Bale was completely joking a few days ago at a press day for 3:10 to Yuma. Bale didn't want to talk about "Dark Knight" (understandably so, considering the press day was for another movie) and threw that out as a joke.
And if you believe the reports that Bale never jokes around, you can watch a clip here of him and Crowe talking about the Justice League of America movie.
Source: Superhero Hype!