As found on
Cube-Europe
The SD Digi-Card Adapter is already available for purchase from Nintendo's Online Store (Japan). There are 2 "versions" you can buy. One with just the adapter, and the other the adapter plus an SD card. The Japanese release date of the SD Adapter was supposed to be July 18th, but maybe you can buy online now, and the actual store release date will still be July 18th.
The Japanese and their SD Digi-Card Adapter
Threaded Mode
