Nintendo's confirmed it's working on a system to transfer Wii save data, WiiWare and Virtual Console purchases from your Wii console to the new Wii U.
The good news came during last night's developer roundtable in a Q&A section, a Nintendo producer saying:
We are readying the system to be able to transfer saves and everything you've bought.
The producer didn't say anything about the logistics of the transfer — so we don't know if you'll have to keep your Wii console to send your content to Wii U — but at least Nintendo's working on it.