Get a white Wii so you can play it now. Then get a black case down the line I guess, I'm sure there'll be high quality replicas of the black case for us white Wii-ers. Or even a broken black Wii console one day from ebay, you could change the cases
Theres a thread about Wii Skins, and there's a black skin for the Wii, so if you really care about getting a black one, get the white one now, and buy a skin