Polygon has posted how the transfer between the two systems will work
http://www.polygon.com/2012/11/7/3613798/nintendo-outlines-wii-to-wii-u-data-transfer-process
Also shows what can be transferred over.
The process is somewhat similar to the transfer process between Nintendo's handheld devices, but requires a couple of extra steps. First, users must insert an SD card into the Wii U to initiate the process using the console's Wii System Transfer application. After starting the process, the user must put the same SD card into their Wii, and download another transfer application from the Wii Shop Channel. That app will move data over to the card, which must be put back into the Wii U, at which point the Wii data is moved to the console, and deleted from the card altogether.
http://www.polygon.com/2012/11/7/3613798/nintendo-outlines-wii-to-wii-u-data-transfer-process
Also shows what can be transferred over.