This seems like the worst kept secret yet, but Mortal Kombat vs. DC Comics seems official according this poster even though the url there doesn't work for now. More details will surface tomorrow or in a matter of hours, but for now, this is nuts...
UPDATE - Here's the announcement trailer with some gameplay revealed as Ed Boon mentions "Klose Kombat" and "Freefall Kombat" as some of the new features to the fighting. Then again, this is still crazy stuff.