The Intelligent Surveillance and Guard Robot is the first fully robotic system to offer surveillance, tracking, firing and voice recognition built into the same device.

"Until now, technology allowed these robots to conduct monitoring function[s] only. But [now] our robots can detect suspicious moving objects, literally go after them, and can even fire at them," said Sang-Il Han, principal research engineer at Samsung Techwin.

This robotic sentry gun is able to use voice recognition to check on answers to the "Who goes there?" query. If the intruder is unable to provide the necessary access code when at a distance of ten meters, the Intelligent Surveillance and Guard Robot can either sound an alarm, fire rubber bullets or make use of its K-3 machine gun.

Prototypes will be tested this year; deployment along the border could take place as early as late 2007.

There is an early and very well-visualized instance of a robotic sentry gun in Michael Crichton's 1969 bestseller The Andromeda Strain. In the novel, a group of scientists are working in an underground laboratory with seven levels; the levels were connected by an open central core.

Stone was sitting in the Level V laboratory, watching on the consoles as the computer electric eyes picked up Hall and outlined his body moving up the wall. To Stone he seemed painfully vulnerable. Stone glanced over at a third screen, which showed the ligamine ejectors pivoting on their wall brackets, the slim barrels coming around to take aim...

On the screen, Hall's body was outlined in red on a vivid green background. As Stone watched, a crosshair was superimposed over the body, centering on the neck. The computer was programmed to choose a region with high blood flow...
(Read more about Michael Crichton's automatic gun)

Also, be sure to take a look at an earlier (and homebrew!) example of this technology - the Robotic Sentry Gun From USMechatronics.

The earliest examples of automatic guns or self-aiming military weapons systems are from WWII; the SCR-584 (short for Signal Corp Radio # 584) gun-laying radar. It could detect bomber-sized targets at about 40 miles range, and was generally able to automatically track them at about 18 miles. Accuracy at this range was 25 yards.

Update 08-Oct-06: Take a look at the sentry guns from Alien 2 included in the comments to this section.