This movie would be a massive undertaking and unless the studio is off it's nut it would have to bring in Christian Bale to play Batman and Brandon Routh to play Superman. And that's just the start of it. Think about the budget for a film that needs to showcase at least half a dozen well-known superheroes and the level of catastrophe and villain that would be worthy of engaging them!
I also looked up the writing credits of Kieran and Michele Mulroney, the two folks tasked to take on this massive project and all I found are two (apparently small) films that are currently in pre-production. Not the most confidence-inspiring background for a film of this type.
I think that the only way that this might be even remotely feasible would be if they would run with the concept and characters that they've developed in Smallville, but I doubt that's what they're thinking.