It's not unusual for a film that the producers think will be successful, to have elements that could lead to a sequal.
But a sequal will only be made if the film brings in more money than it took to make.
Have recently read that there will be a sequal, the movie may not have performed 100% at the box offive but come November / December the DVD sales will surely be good - news is the sequal will have General Zodd (from Superman 2) as the bad guy!