The Wii U can transfer data from an older Wii using a new channel called the "Wii U Transfer Tool" which Nintendo has made available. However, the instructions state that once the data is transferred off the Wii to the Wii U, the data is permanently deleted from the Wii and cannot be restored. According to the Wii U Manual:
IMPORTANT: Transferred data is permanently deleted from the Wii Console. Once you have Transferd data to the Wii U Console, it cannot be transferred back to a Wii console. Data cannot be transferred from your Wii U console to a Wii console.
I just tried copying all my Miis to a Wiimote, but on the Wii U, There is no way to copy Wiis from a Wii Remote back to the Wii U... so that plan failed...
Is there any way to transfer the data from an older Wii console to a new Wii U without deleting the data off the old Wii? I don't trust the new Wii U to be 100% backwards compatible and I might still want to use the Wii!