I predicted this two days ago, i.e.:
The Wii "Board" is guaranteed to sell out for forever (or until hell freezes over, or they make a better one). It will sell at least five to every nursing home in North America, Japan and Europe. People will be buying them for their parents to try and get them to move around some more and, not surprisingly, for themselves. It will be used in successful FPS's, sports games, rhythm/dance games (duh), a new Mario Party and a ton more. It will be used by doctors, physical therapists, sports trainers and dietitians.
"The Board" has so many potential uses. Trying to think a little outside the box - for shooter games you could tap your foot twice to reload. Mario games could substitute the "Z" button with a little hop on the board. Driving games are obvious with more pressure on one side to accelerate, the other side to brake. Besides the strafing slide for FPSs it would be great in a game like Sports (and make it more skillful) to have to shift your tennis player from side to side, or align the bowler by keeping in balance, of for golf measure the fade or hook and power by balance in addition to the motion of the Wiimote. For ANY game it could still be used as two more buttons. It could substitute for the L and R or the X and Y buttons. "The Board" still obviously makes more sense with shooters, platformers, sports, etc. but it is potentially compatible with any game.
The reason Nintendo featured WiiFit and the so called "accessory" that goes with it isn't just because they want to sell a fitness and yoga trainer, it is because they want "The Board" to become a part of the basic setup for at least a plurality of the lineup. They're still innovating and differentiating. Stay tuned, Wii'll see if it works!
