During the Nintendo Direct Conference held on the 27th of December, Iwata revealed that the first Nintendo 3DS game to support Downloadable Content (DLC) would be Fire Emblem: Awakening.
No details were giving about what the DLC would include but chances are it will allow players to download new maps, items, weapons and possible even new characters.
Downloadable Content support was added to the 3DS in the "December Update" but no game currently supports it.
This page will be update once we know more about the DLC