So little bit of news, the japanese website actually says thew following, according to translators on serebii;
"Pokemon which have undergone Mega Evolution return to their normal form when the battle ends, however they cannot return to their normal form during battle. Also, only one Pokemon may undergo Mega Evolution per battle."
So it's going to be a kind of championing thing, where you pick one poke to be your mega. Personally, I think this is pretty cool. Though I am worried. If not all pokemon can be megas, what if your favorite poke can't be your 'champion'? I'd love it if sandslash got a mega, but I doubt he will, for example.