Recruit, which is one of the major magazine publishers did a survey on 1,000 internet users who are 20-34 years old.
In the marketing world, 20-34 old male are called M1, 20-34 old female are called F1, so the result of this survey was called "M1F1 Grand Prix".
M1F1 Grand Prix results for January-June 2008 is as follows:
Data Source: Hot Pepper (Magazine by Recruit)
Ranking till the 20th can be seen here with the photo from the awarding ceremony.
You can see that Wii Fit won the 1st place both in M1 and F1.
Many Japanese people loves games, but I think Wii and Wii Fit opened the door to the non-gamers out there to start playing games.
Photo CC-By-NC by Nataliej
This Japanese programmer - Yappo - has created a PC controller by hacking wii fit and presented at YAPC Asia (Yet Another Perl Conference in Tokyo). Very cool :)
Photo CC-BY-NC-SA by Fumi
Following is the data of weekly sales of games in Japan.
Data source: Famitsu.com
Wii Fit is ranked 5th now, but the total sales is incomparable from other games.