It appears that users are being banned from 3DS online services, en masse. There's no direct cause right now, and seemingly no linking factor. This post is meant as an announcement for users, to be wary, and that GBAtemp will keep you updated as soon as more info rolls in. If you have been banned, please post in the thread below, and we will try to keep tabs on the situation. To be extra sure, it can't hurt to turn off your 3DS's wireless...
UPDATE: Regarding the recent 3DS banwave