Yeah, apparently there was a trailer with Heracross vs. Darkrai along with the Black and White trailers that revealed Minezumi and Victini.
And hey, what's to say that Ash can't beat a Darkrai? He's beat an Articuno and a Regice before, and put up good fights against Deoxys, Regirock, and Registeel. Granted, it will probably take three or four Pokemon to bring Darkrai down, but I think that Ash could pull it off. After all, since Takuto has used Darkrai alone in all of his battles, it can be assumed that he'll lead with it again. I'm sure that if all six of Ash's Pokemon come up against Darkrai, he can bring it down.