To clear up the confusion, in the recent edition of Empire Magazine, Zack Snyder and his wife (and also producer), Deborah Snyder confirmed that the Justice League movie depends on the success of Man of Steel. He also confirmed that if it does well and that Warner Bros. wants to greenlight the project, he will be asked by the studio to direct Justice League.
Snyder did not specifically say whether or not he will accept the job though. After the release of Man of Steel, everyone will find out shortly enough.[via Geekscape]