It's called Recochoku (short for "Record-Chokusetsu" or "Record Direct"). And it looks to be Nintendo's take on iTunes.
The service begins this December in Japan, and there will be over a million songs available for purchase. You can purchase songs via the 3DS, but you must be a member of Club Recochoku to purchase songs for Recochoku.
It seems to essentially be a cloud based service. Songs that you purchase on Recochoku can be redownloaded to other portable devices, such as smartphones.