They were going to do it in 2004 with Wolfgang Peterson as the director but it got scrapped. The possibility of Nolan's Batman and Singers Superman coming to together for a movie was brought up, but Nolan didn't want it to happen. So then WB decided to make a Justice League film, called Justice League: Mortal that would have had both Batman and Superman, been directed by George Miller and come out in 2010. It would have been completely separate from Nolan's Batman universe. However the film was scrapped in 2008. So they actually have been trying to get the two characters to crossover for a long time now.