I had actually been sitting on this rumor for a while as I couldn't second source it, and it may still be just talk but The Playlist have evidently given some(small) credence to it too.
While discussing the recent news that Viggo Mortenson will definitely not be starring in the movie they offer the following..
..It’s a shame, as Mortensen was a solid choice for Zod, but we’re sure that it won’t take long to fill the part—vague whispers have suggested that Snyder wants to reunite with his “300” lead Gerard Butler in the role..
They may just be the same "vague whispers" I have heard but with those final few major roles needing to be filled fairly sharpish it's not too far fetched to think that Snyder would be looking at actors he has worked with in the past. Particularly actors with such a (potentially) intimidating physical presence as Butler. For his part, Butler has settled into rom-com territory recently and could well see this as a chance to get back in to some more action orientated, meaty roles.
IF any of this is even accurate of course. As it stands we don't even have confirmation that Zod is the villain! Anyway, what would you think of Butler for the role if that was how it transpired?