Although the secondary port on the Nintendo DS does accept and support Game Boy Advance cartridges (but not Game Boy or Game Boy Color cartridges), Nintendo has emphasized that its main intention for its inclusion was to allow a wide variety of accessories to be released for the system, the Game Boy Advance compatibility titles being a logical extension.
Nintendo announced at E3 2005 that it would launch "headset accessories" for voice over IP (VoIP) enabled games. (This will plug into the VoIP plug next to the Ear Phone jack, not the Game Boy Advance slot.)