As far as I can see, there is no Nintendomination.
Nintendo used to have 100% of the handheld market, now they lost 1/3 of that business to Sony's PSP.
Nintendo Wii is doing pretty good on the living room consoles market with close to 45% but it's nowhere close to what the PS2 did/is still doing.
Also, Wii's lifetime is most likely going to be shorter than 360 and PS3 due to it's non-HD characteristics so Nintendo will have to migrate a lot of "casual" users into a new console. It will not be an easy task for Nintendo.