This certainly isn't the end of the world for the Wii. Like it's been said, the logical explanation is that just about everyone has one.
But a lot of folks I know bought Wiis, and they are now gathering dust in favor of playing PS3 or XBox games. Why? I'd have to say it would have to be A)Nintendo's turn away from the core market to the casual one and B)Immense profits from said user base.
While I can't really fault Nintendo, especially during these economical times, with going with the path of least resistance to the dollar, I've always wondered what kind of long term viability the Wii really has. Sure, you can move a lot of consoles, but if the casual mini-games run out and the lack of actual third-party titles causes a lackluster interest, then the Wii may have trouble in a year or two's time staying viable.
Nintendo's only recourse with the Wii now is to move further along the casual game market and build its new core install base with them, because aside from iconic, well known characters, I haven't seen any reason to purchase a Wii game in months.