In the past six years I've been through four Xbox 360 consoles and three PlayStation 3s. Which has been a pain, but each and every time I've been able to simply transfer my existing system account to the new console through the magic of the internet.
You'd think that would be something Nintendo could manage rather easily, but nope. Turns out that the Nintendo Network ID you create for a Wii U will, for now at least, is tied to that console. If you lose that console - for whatever reason - you lose the ID that went with it. If you buy a second-hand console, and want to import your ID, you can't. If you're at a friend's house and want to sign on using your account, you can't.
Here's the relevant statement from Nintendo's support page:
That "future" consoles thing is a little confusing, as it's unclear whether they mean the next console after the Wii U, or simply Wii U machines in the future, when this feature is patched in. We'll contact Nintendo and update if we hear back.
Hopefully it's something that can be patched in, because boy, if people lose their accounts and their consoles at the same time, they'll be pretty damn upset.
Can I Log Into My Nintendo Network Account on a Different Console? [Nintendo Support, thanks everyone!]