A company called Vocera Communications has created a working implementation of the communicators that are seen on Star Trek.
It's actually a rather slick setup, too. You wear a little badge, not unlike those seen on Star Trek. Each badge has a Digital Signal Processor (for sound processing) and a WiFi chip. When you press a button on it, it activates the badge. You then speak the name of the person that you wish to talk to. A central server processes your speech to determine who you want to talk to, then activates their badge and sets up a wireless VoIP (Voice over IP (Internet Protocol)) connection between the two badges.
Apparently their biggest customers are currently in health care, which makes sense. This could (literally) be a lifesaver for nurses and doctors trying to quickly locate or contact one another.
I really want one, though. ;-)