The "Superman Returns" situation is quite different since Bryan Singer had moved on to film "Valkyrie" and really never got a green light from Warner Bros regarding a sequel. There were plans for one and Singer has since revealed what those plans would have been. Warner Bros sat on the franchise after Returns and allowed Brandon Routh's contract to run out (on purpose is my own personal theory). If things had stayed on schedule and a Returns sequel actually happened it would have come out probably sometime in the summer of 2009.
Maybe, but regardless of Singer's schedule or talk of "plans", it seemed pretty clear to me that the WB was never terribly excited or motivated to make a sequel to SR.
I'm sure if they were, Singer (being the huge Superman fan that he is) wouldn't have hesitated to make room in his schedule for it.