A lack of large storage space only impacts downloadable content. (and the GH3 guys are working with Nintendo to figure out something for them)
It would not surprise me to see a Wii version of Rock Band next year. I think they didn't want to be spread too thin for all the hardware that needs to be made for it. But once the intial demand is settling down, I think they'll look seriously at a Wii version, especially since the Wii version of GH3 seems to be selling very well.