Nintendo Network Server Maintenance Causing Extended Downtime Tomorrow
The Nintendo Network goes down for planned maintenance every week, but this week the downtime is longer and effects more Nintendo online services.
From 8 AM Pacific Time on Monday September 30 to 5 AM Pacific Time, online multiplayer, online rankings, private messages, friend requests, blocked-user lists, and other services will be down or sporadically available.
The Internet rumblings surrounding the downtime presume preparations for larger firmware updates soon coming to Wii U and 3DS. This seems very likely, but Nintendo hasn't confirmed any dates for upcoming updates.