@Stranne: Actually music charts between NA and EU as a comparision was pretty bad, since in EU you have a lot more to choose from overall, and when it comes to different languages, there are only the certain amount of people who buy music other than their own language or music in their own language. And games are basically the same everywhere.
@Topic: Nice and civil topic for a change and good points from everyone. I don't think that there's a single reason for dropped sales, but i see "drought of games" and the release of the new PSP being the biggest factor. Japan and NA have had almost equal amount of Wiis, while NA has 2,5 times the population of Japan and NA also has bigger games market. So in order to maintain sales at the same level with NA, you have to have "killer apps" constantly. Since Wii doesn't yet have a solid games library, which itself would drive sales, it needs new games that can sell systems.
And the PSP, despite the fact, that Wii and PSP doesn't compete, there's still a lot of people who buy both of them and there's no particular reason to get Wii now since SMG is yet to come and PSP, with solid games library, just got a new fancier version. You can anyway play PSP until you get Wii with SMG.