It now seems as if it IS possible to transfer Nintendo Network IDs from one Wii U to another.
Until now official Nintendo guidelines had suggested that once created a Nintendo Network ID is locked to a single machine and cannot be transferred to another under any circumstances – even if users are replacing a faulty machine.
The Wii U FAQ reads: No, you cannot [log into a Nintendo Account on a different console]. A Nintendo Network Account can only be used on the console where it was created. In the future, you will be able to use your Nintendo Network account with future Nintendo consoles and other devices, such as PC.”
However, a Nintendo of America rep told Kotaku that: "NoA’s repair process includes the transferring of the Nintendo Network ID to a new Wii U console if a replacement system is needed."
This does mean, of course, that gamers are prevented from simply swapping faulty machines at the point of sale. If they wish to retain their ID, Nintendo must handle the repair directly.
This official confirmation seems to contradict previous reports where consumers were told by Nintendo reps that it is simply technically impossible to transfer Nintendo Network IDs from one console to another.