Apparently Nintendo is blaming the economy for the drop in Wii sales.
I'm sure that's part of it. But the less than glittering truth is that Wii sales benefited from gamer backlash over the initial price tag of the PS3. In fact, the slogan "Wii 360" was adopted by gamers who decided to only purchase Nintendo's Wii and Microsoft's Xbox 360 due to what was seen as exorbitant pricing on Sony's part. Now that the price of all three consoles has dropped, the Wii is basically left pushing its novelty which is backed by a generally unimpressive selection of gaming titles. Unfortunately for the Wii, its novelty is self defeating when it comes to most of the standard game genres- random stick waggling is a less efficient way to navigate action/adventure, fighting, and standard (non-Wii sports) model sports games than random button mashing.
Sorry, Wii. You were always destined to be a niche market product. It looks like you're finding out that niche wasn't a large as you thought.