Ethernet Intercom Systems?

Milo_Mindbender asks: "Has anyone seen an intercom system that operates entirely over Ethernet? Ideally, I'm looking for something like they use in TV and theatre, a self-contained belt-pack you plug a headset in one side and an ethernet cable (or wireless card) in the other. It needs to be controlable by a master computer that can link each user to any number of channels and change them around on the fly. A channel should be full-duplex and allow multiple people to talk/listen at once like a telephone conference call. This is for an entertainment venue where guests will use the system as a 'radio simulator' while playing multiplayer games and the staff will use it to help manage the facility. I'd prefer a ready-made system, but if you have suggestions for hardware/software that could be converted into such a system I won't complain!" While something like this would be a fun thin to do, why would someone use ethernet when radio-based (or, if you are careful...wire-based) communication products are cheaper and more robust?