Second Talk, is a skype-based voice chat system rolling out a public beta right now, courtesy of Centric. Stop in at once of Second Talk's inworld kiosks, and grab a Second Talk headset. Put it on your avatar. According to the documentation, whenever you're in range of others wearing the Second Talk headset (and presumably not anywhere set no-script), you'll be prompted to see if you want to talk to them.You'll need a skype account to make this all work, and instructions on plugging your skype details into the headset are provided with the instruction card. Second Talk are looking into integrating with the open source client. I was unable to actually try this out myself due to certain personal incapacities relating to voip communications.
