Capcom Associate Online/Community Specialist Yuri Araujo posted on the Capcom-Unity thread about the issue, apologizing for the mysterious delay and stating that it would be up late yesterday. As of writing, it is not. We've checked in with Capcom for an updated ETA.
For now, you can play Monster Hunter 3 Ultimate on 3DS or Wii U – you just can't play on Wii U if you want your progress from both to carry over.
Update: "Capcom is working closely with Nintendo to make these available for download as soon as possible and apologize for the delay," the publisher tells Joystiq.