It is being reported that the Nintendo DS has a headset port which will be used in conjunction with the built-in wireless 802.11b networking capabilities to offer VoIP chat. This will also enable gamers to use the Nintendo DS to make free phone calls or simply to talk trash while fragging your opponent.
Of course Microsoft Xbox Live already does this. But rumor has it the Nintendo DS will enable PSTN phone calls - not just gamer-to-gamer voice chat.
I did some research and couldn't determine whether Nintendo will be using someone like Vonage to power their PSTN capabilities or Skype or someone else. Knowing Nintendo's penchant for proprietary game cartridges, I wouldn't be surprised if they use their own proprietary VoIP codecs and perhaps their own proprietary means of hopping off to the PSTN.
Of course, they could save themselves a few billion and just use Level3's excellent QoS managed VoIP network rather than build their own.