Voice over IP with the Nintendo DSIGN has some brief but tantalizing coverage of a new demo called DSpeak. The phrasing of the article makes it sound like this is working, but not slated for any known game.
"Both systems are connected to each other using the WiFi internet capabilities of the Nintendo DS system. The demo is incredibly simple: using a connected headphone/microphone piece, you speak into the microphone and listen with the earpiece. You essentially hold conversations using the Nintendo DS over the internet. The demo features a 3D Nintendo character on the upper screen, with a wavelength meter displayed on the lower screen. On my screen was Mario, on the other guy, Wario."