The next Superman movie will be a reboot rather than a sequel to Superman Returns.
Warner Bros. Pictures Group President Jeff Robinov told the Wall Street Journal:

“‘Superman’ didn’t quite work as a film in the way that we wanted it to…It didn’t position the character the way he needed to be positioned. Had ‘Superman’ worked in 2006, we would have had a movie for Christmas of this year or 2009. But now the plan is just to reintroduce Superman without regard to a Batman and Superman movie at all.”
Source: http://online.wsj.com/article/SB121936107614461929.html

(also ruling out any crossover movie).

“We’re going to try to go dark to the extent that the characters allow it”, Robinov adds.