Kind of a weird way to look at things, but interesting at the same time.
There were 14 million WiiFits shipped last year. (gamasutra)
Every copy came with the Wii Balance Board.
There were 10.8 Million Xbox 360s shipped last year.
Total 360 distribution sits at 28.45 million shipped. (gamasutra)
So, taking a lookt at that, I would say that it seems well within reason that within the next two years (one?) more people will own a Wii Balance Board than will own a 360.
That's just amazingly impressive and I think underlines just how well WiiFit is selling around the globe.