The game Wii Fit will sell, no doubt there, but not to the hoards of non-casual Wii gamers. I don't have a Wii but housemate does so play it, when I mentioned it to him, the only aspect that appealed to him was the board itself because of it's use in other games (using as skateboard or even pedals on a car, it's jsut pressure pads after all). I think many will buy it for the board alone.
So, I think the board will be the successful peice of hardware not Wii-Fit the game (much like I think Wii Sports is), like Eye Toy was at first, but over time, limited games for it will restrict it's use.
Hmm, pie.