The first arc of [Justice League] takes place 5 years in the past and serves as a Year One story for the Justice League. In story, up until this point, there have been no 'super-heroes', just super powered individuals who have sometimes been viewed with suspicion. By banding together, the Justice League wants to show the world that they are deserving the moniker of super-hero.
The first arc will detail Superman's public debut. At this point, Dan Didio went on a tangent to explain that DC wanted to let go of the past and move forward. Any changes in continuity would be addressed in-story as needed, but that they really didn't want to get bogged down so these reveals would be gradual and as they make sense in the story. There is a document in the DC offices that fleshes out exactly what has and hasn't happened in the New 52 Universe. As an example, the Death of Superman storyline has occurred.