Nintendo has never been the best about creating user friendly online systems. Remember friend codes? Yeah, not the best plan in the world. On the upside, the Wii U does away those annoying friend codes and gives players a Nintendo Network ID instead. Unfortunately, there is still a bit of a catch; your Nintendo Network ID is, in fact, tied to the console you purchased. For now, there is no way to sign onto a friend’s console with your NNID, nor is there a way to transfer your Network ID. So, should your console break, you’ll have to start the process from scratch.
Nintendo has said “In the future, you will be able to use your Nintendo Network Account with future Nintendo consoles and other devices, such as PC's,” but for now “A Nintendo Network Account can only be used on the console where it was created.”
Nintendo has not yet confirmed whether or not they will eventually let you transfer your Nintendo Network ID to other Wii U consoles. Hopefully they will. Otherwise, if your console breaks you will lose all your saved games, all your online activity and friend's lists. Plus, and perhaps most importantly, players would lose access to all of their e-shop purchases.