The idea of a Justice League movie is both exciting and difficult to imagine because it would consist of more superhero star power than audiences are used to (Marvel has a similar idea going on with a possible Avengers movie). One big question sure to be driving DC Comics fans mad is will Batman and Superman appear in the contexts of their respective franchises, or in other words will they be played by Christian Bale and Brandon Routh? Maybe Warner Bros. is even thinking of doing the JLA movie in order to launch the franchises for the other four characters (so far we've seen no love for Martian Manhunter at the studio).
My guess is the WB is hoping for a 2010 release, following The Dark Knight in 2008 and Superman: The Man of Steel in 2009. Then, they will get to the spin-offs. And eventually, I'm hoping, there will be some sort of crossover movie that pits the JLA against The Avengers. See, I just can't ever be satisfied. As long as the studios keep producing my dream comic book adaptations, I will have to keep coming up with bigger dreams.