In today's installment of "Predictable, but disappointing," it's revealed that in order to move data from your Wii to your eventual Wii U, you'll need both systems at the same time for the transfer process. At least we found out before everyone went and traded in their systems, I suppose.
After reaching out to Nintendo, Gimme Gimme Games received the following response from a rep:
I can confirm that you will be able to transfer your content from your Wii to the new Wii U. Wii U owners can wirelessly transfer content such as Miis, and WiiWare and Virtual Console games to their Wii U systems. Both systems will need to be connected to the Internet and the Wii U will need an SD card to facilitate the transfer. The process is very similar to transferring content from Nintendo DS to Nintendo 3DS or from Nintendo 3DS to Nintendo 3DS XL; which has been received positively by consumers.
Again, most of us probably could have seen this coming, but it's worth pointing out if only to save a single person from making an unfortunate mistake.
Don’t trade in your Wii yet! Nintendo confirms that you’ll need both your old Wii and Wii U to transfer data/games between systems [Gimme Gimme Games via Go Nintendo]