Another problem for Nintendo is that quickly ramping up production often leads to poorer quality products. Nintendo would then have to either play Microsoft and just ship broken (or about to break) Wiis or take the financial hit of higher QA costs and lower production. It's nothing insurmountable of course, it just takes time.
Something else I wonder is who all produces the accelerometers for the Wii? That seems like it could be a possible bottleneck to increased supplies. Doesn't matter if Nintendo can get Samsung, IBM, ATI, MoSys, etc to increase parts production if they can't make anymore controllers.