Nintendo may be planning to release an official Wii pedometer peripheral - emulating UK studio New Concept's Wii Jog device.
Wii Jog, on the other hand, is also a square bit of plastic, priced GBP 14.99 (down from GBP 34.99), and dated for late October on Play. This measures footsteps taken and translates that into Wii games.
Demonstration footage shows leads from the prototype box plugged into the Wiimote and nunchuk, while the box itself is placed in the trouser pocket of a smartly-dressed model. When the model jogs on the spot, the character in-game also runs. Witchcraft!
Wii Fanboy has some videos of Wii Jog in motion, and a PS2 version is already available from Amazon in limited quantities according to New Concept's PRs.