Looks like you'll have to hang onto your Wii if you plan on transferring your content over to the Wii U.
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. Transferred Wii content is accessible and can be played through Wii Mode, an application on the Wii U’s main menu. - Nintendo rep
Since we're dealing with a console, this might make trade-ins a little bit rougher!