Designed to be used with games like Metroid Prime: Hunters and Pokemon Diamond & Pearl, the headset lets you talk to other players through Nintendo's WiFi enabled DS. The headset goes into the little hole next to the headphone port.
The accessory is going to cost 1200 Yen ($10), available September 14. With all the DS homebrew solutions, someone's bound to put some kind of VoIP client like Skype or a SIP-compatible one on there so you can make calls on your DS.
Nintendo announces headphone VoIP set for DS [Video Games Blogger]