The 3DS gamecards have the same physical interface as regular DS and DSi gamecards. There is only a minor cosmetic difference in the plastic case, which has a small extruding notch on the top-right side. As such, it prevents insertion of the gamecard into old Nintendo DS or DSi systems.
When modifying the case so that the 3DS gamecard fits in a DS or DSi system, those systems will refuse to detect the gamecard and show no banner icon.
The communication protocol between the 3DS system and the 3DS gamecard has changed completely in comparison with the DS and DSi gamecard communication protocol.