The X360 and PS3 are quite easy to predict, but the Wii is hard.
Will it be like the DS? - technically outdated but still selling year after year.
I believe so. There won't be a Wii 2 before Sony and MS have launched their next gen consoles (nintendo won't risk to kill the Wii prematurely in any way).
The Wii sold 26 million units in 2008. I think that is near it's peak, but it will maintain high level of sales for years to come.
X360: 55 million
PS3: 55 million
Wii: 150 million
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260 million