Wow the link Taurus posted harboured this:
Merrill Lynch, Wii household penetration in US/Japan through 2011, made February 2007
http://biz.gamedaily.com/industry/feature/?id=15309external link
Now Merrill Lynch analyst Yoshiyuki Kinoshita is forecasting that by the year 2011, nearly 30 percent of all U.S. households will own a Wii. In Japan, Kinoshita believes that number will climb even higher, reaching a third of Japanese households.
---
The question I have to ask is how many households are there in USA and Japan?
I would suspect about 100 million in USA and 40-50 million in Japan, (though I have no idea) This would mean over 30 million Wiis in America and 15ish million in Japan.