Ata developers' roundtable after E3 today, two of Nintendo's top designers confirmed that "everything you've bought" will transfer from Wii to Wii U along with save files. But they did not reveal how. Stay tuned!
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Prof. Bananas Goldsteinberg
So now, the only reason for me to keep my Wii once the Wii U launches, is to use it as a Gamecube.