@jarrod: the fact that Wii fell behind PS2 shows how dramatically it declined.
Now, lots of consoles have had weak years for software, but what's happened in Japan (and has been happening since launch) is pretty much unprecedented and most analysts (including Iwata) think it has to do with Japan becoming bored of the Wii tech. Check out this link which shows the 12-month moving average through April of this year. (For the rest of 2009, the Wii line would have been in constant decline until MH3 released at which point it would have gone up from 1.72 million to 1.77 million per 12 months, then declined again until the Wii price cut at which point it would have started to increase slightly to just above flat.) At the point when the graph ends (April), Wii was at "2.2 million for last 12 months" whereas it now sits at about "1.75 million for last 12 months". For reference, PS2 was doing "3.9 million for last 12 months" at this point in its life cycle (just after its peak). PS3 was at around 1 million when the chart was done, but is now at 1.38 million and increasing sharply.
The point is that the behaviour of Wii in Japan shows a sales trend that I don't think has ever been seen before anywhere. You say yourself that consoles tend to hit a peak which is true. But Wii peaked in Japan during its first 12-months on the market and (I think) other than Mario Party 8 and SMG, its biggest titles didn't come out until after that.
Lots of people keep saying the Wii's decline is all about software, but there's obviously bigger momentum issues at play.
http://dubiousquality.blogspot.com/2009/04/console-post-of-week-japan.html