They won't take off like the Wii has but they will boost sales as the consoles come down in price. This is especially true for Microsoft. A sub $199 360 with Natal and a game that can't be played on any other system that gets people excited about it would give the 360 a nice boost and stave off the need for a new console by perhaps a year.
For Sony it's not going to work out at all.. Price is still too high and it's basically a Wii Motion Plus but without the big mainstream franchises and no back catalog up against Nintendo's 3 years worth of games. Microsoft are at least differentiating themselves a little bit.
I think the thing that really stands out is the fact that the Wii Balance Board, having a market penetration coming close to the PS3 but without very many games is telling of the situation these new peripherals will find themselves in. When the 720 comes with Natal 2 launching with Halo 4 (Natal compatible or even Natal only) then we'll start to see these technologies taking off. Until then this generation has already basically been decided.
