The Nintendo Network is scheduled for "emergency maintenance" starting tonight and lasting through tomorrow afternoon. The maintenance is said to impact all of the online shops across its various platforms, rankings in the Pokemon Black/White and Black/White 2, and ranking and matchmaking functions for Warriors Orochi 3 Hyper.
The schedule (via Polygon) also notes that "parts of the affected services may be intermittently unavailable" during the maintenance. What it doesn't detail is the nature of the "emergency" that has prompted the maintenance. Currently all services are operating normally, and will continue to do so until 11 PM Pacific.
Shacknews has contacted Nintendo regarding the maintenance notice and will update as more information becomes available.