The Nintendo eShop has been drifting in and out of service since sometime on Christmas Day, Kotaku reports, but it's finally back online as of earlier this afternoon. The eShop outages were due to a somewhat predictably large volume of traffic on the biggest gift receiving day of the year. The traffic jam was exacerbated by the creation of new Nintendo Network accounts and conversions from 3DS accounts.
The intermittent outage lasted about four days. It directly overloaded the 3DS and Wii U eShops, but the effects extended throughout the entire eShop service. During that period, gamers couldn't create new Nintendo Network accounts IDs.
Another factor in the outage was the ongoing mass conversion to Nintendo Network accounts. Until recently, 3DS gamers had to set up separate accounts. Now Nintendo is having everyone switch over to the more unified Nintendo Network.
As a result of all this, Nintendo still hasn't launched the Pokémon app titles Bank and Transporter. The apps were scheduled for release on Friday, but the outage was still ongoing at the time and Nintendo didn't want to compound the traffic surge, so it postponed the launch until a later date. That date has not been specified.