Nintendo, Sony and Microsoft generally start development on the Wii years before it's actually released. We won't see this new console for a few years yet, at least.
As for whether the Wii 2 will be a major upgrade or a smaller upgrade a la the GCN->Wii, lord only knows. It's way too early to tell. I think it'd be a bigger upgrade, Nintendo IMO can't really afford to do a minor upgrade for the next generation, it'd be too big of a gap between the other two (it's too big of a gap now, but that's besides the point).