Japan is the last thing to worry about, imo. It may be possible that DS´s huge success won´t turn Wii into a huge success, but at least Nintendo is the "cool thing" in Japan right now. When Nintendo recently started Wii´s marketing campaign on Japanese TV, only hours later people on the streets were talking how Nintendo has a new console coming out. That´s a pretty good sign, don´t you think?
If you want to worry about something, worry about NA or Europe because that´s were Wii is more likely to fail. But, it´s also possible that there might be slightly more important things in life to worry about than how well some game console sells in some country.