Take this one with a HUGE grain of salt, folks, but the occasionally-news and rumor site Latino Review is reporting that Will Beall's script for the upcoming Justice League movie will be based on a Silver Age story by Gerry Conway and George Perez...and not on the first arc of the New 52 Justice League as has been widely expected by fans.
These rumors sync up with earlier rumors, also started at Latino Review but widely repeated throughout the entertainment press, that Darkseid would be the film's villain; the storyline in question took place in Justice League of America #183-185 and features the Justice League and Justice Society of America taking on Darkseid, who wants to move Apokolips into Earth's place.
The issues are currently available in the trade paperback collection Crisis on Multiple Earths Volume 5, or on ComiXology for $1.99 apiece.
Latino Review also suggests, as most are assuming, that the film remains without a director. Whether they will have one in place or not before the project is formally announced is anyone's guess. It seems unlikely at this stage that a formal Justice League announcement will come along before Man of Steel is in theaters, in any event.
While Marvel Comics has up to this point resisted the urge to adapt specific storylines for their films, Warner Brothers's success with The Dark Knight Rises this year, which drew heavily on Knightfall and No Man's Land, may have been a proof of concept for the studio to decide it's a good way to go forward, limiting the screenwriting time somewhat because your plot structure is already in place for the writer.