Update Almost all services are back up and running
Update (1/29/13): A quick peek at the Nintendo website shows that service has been restored to the Wii U, Wii, and 3DS platforms. The Nintendo DS service status remains the only exception, and reads: "Some parts of the online services are experiencing technical difficulties."
Everything else has been given the blue circle of approval.
Nintendo has taken down their Nintendo Network service today, for what the company calls "emergency maintenance." The service is, however, expected to go back up by tomorrow, January 29.
During this time, certain services will be unavailable, like the online Wii U and 3DS eShops, the DSi Shop, and Wii Shopping Channel. If you have a game that utilizes the network, such as Pokemon Black/White and Black/White Version 2, their online functionality will be temporarily unavailable, which tends to encompass ranking and matchmaking.
No details surrounding this emergency maintenance were revealed. Nintendo commented rather plainly, "During the emergency maintenance, parts of the affected services may be intermittently unavailable."