While you might think this is a no brainer, it's important to say out loud that if you trade in that Wii console prior to the launch of the Wii U on November 18, you will lose all of your Virtual Console games and Miis. The reason this needs to be said is because retailer GameStop is offering $50 off the price of the new console (via pre-order) to anyone who trades in their Wii system.
While the Wii U is backwards compatible, you will still need to have both systems in order to transfer all of your data and play those games you have already paid cold, hard cash for.
"Wii U owners can wirelessly transfer content such as Miis, and WiiWare and Virtual Console games to their Wii U systems," a Nintendo representative said. "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 will be similar to transferring content from a Nintendo DS to 3DS, or 3DS to 3DS XL, which also require both systems to be present."
While it would be nice if there was some way to back up the contents of your system onto some kind of portable media, for now you'll have to hold on to your Wii...
Source: The Escapist