...doesn't actually exist (sorry!) - but Nintendo needs to do basically exactly what Microsoft did when they released bigger HDDs. Anyway, they just need to release a USB A to USB A cable coupled with 2 DVDs (one for each Wii) that lets you move your content from your old Wii to your new Wii. It would install a file on both conoles - one for the old one that let's the Wii know that it's data has been moved to the new Wii, so that it cannot be transferred again, and one for the new Wii to let it know where the data came from originally, so that it can't receive data from another Wii. They could also implement the same online system where you can do this once a year (without calling customer service) and you could also redownload all of your Wii Shop Channel purchases this way (connect with the old Wii, make a request on the Wii Shop Channel page setup just for this process - it would give you a code to right down, then you turn off that Wii, connect the new one, connect to the Wii Shop Channel and input the code you were given, and voila - you get your purchase history transferred to your new Wii).
It's really rather simple, they just need to get on it.