According to this MTV article, Harry Potter and the Half-Blood Prince executive producer Jonathan Bunney has confirmed that Wii is the lead format for the game.
“We made that decision from day one,” he said. “We did, I think, a good job with the Wii last year, but we’re doing a great job this year. Because this is where the casual audience is, the mass audience is here. It’s a great control system, but you’ve got to learn to use it.”
More through the link.