I read that it was about $10 million, but that may have been for the ad campaign (TV/video spots), not the entire marketing campaign to include sponsorships, fast food/Mountain Dew tie ins, promotional items and promotional events.
But somehow I don't see there being a WiiFit McDonald's/Burger King tie in campaign.
That $40 million marketing campaign budget for WiiFit seems a suspect number, and not just because Nintendo has never dumped that much into marketing a single game.