It's called Rekochoku (レコチョク). That's 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 Rekochoku to purchase songs for Rekochoku.
It seems to essentially be a cloud based service. Songs that you purchase on Rekochoku can be redownloaded to other portable devices, such as smartphones.
Source:
http://kotaku.com/5954797/nintendo-just-unveiled-its-own-itunes