Nintendo confirmed Game Boy and Game Boy Color titles will be downloadable.
At their press event today, Nintendo announced that the 3DS will feature an online store similar to Wii's Virtual Console. Titles for the Game Boy and Game Boy Color platforms will be available for download, and Nintendo used Super Mario Land and The Legend of Zelda: Link's Awakening DX as example titles for the service.
Nintendo will also be selling "Classic Games" that feature 3D enhancements. While they were mentioned alongside the handheld Virtual Console, few details were offered, and it is unclear if these games are download titles. It is likely that these are similar to the 3DS Classic Collection game demos that were available at E3.