Turning Your PC Into a LAN-based Intercom?

AugstWest asks: "With all of the VOIP projects all over the net, and more and more of us geeks installing machiens in every room in the house, it hit me that we've got all the wiring and hardware necessary for a full-house intercom installed. Software, however, is another story. None of the VOIP projects I've been able to find can be easily adapted to instant-on lan voice communications. With a microphone hooked up to all of the machines throughout the house, shouldn't it be simple to set up instant-on voice software? I've scoured the net for it, does any already exist?"