IESB reports that a script for a possible Batman vs. Superman movie is in the works, and the movie will go ahead depending on whether or not Batman Begins and Superman Returns are successful. I think we can all assume they will. Should the movie go ahead, Christian Bale and Brandon Routh will reprise their roles of Batman and Superman, respectively, as they are contracted to do so.
Here's a basic outline of the plot:
When Bruce Wayne goes out seeking bloody vengeance on the murderer of his new bride, Jennifer. Superman opposes him, fights him to the near death - then it's discovered that the guy behind Jennifer's murder is also this big baddie in Superman's circle and they unite to fight. Warner Bros. has commissioned a script from Andrew Kevin Walker ('Seven') with Wolfgang Petersen ('The Perfect Storm') directing. This is a different project from either Batman 5 or Superman 5.