The fighting in Iraq is more like Vietnam than most people imagine, but in ways that rarely show up on the news. One bit of Vietnam dj vu is the use of the REMBASS (Remote Battlefield Sensor System) family of ground sensors. During the Vietnam war, the use of remote sensors (microphones or other devices that transmitted what they picked up to distant listening stations) became common. The sensors and radio equipment had gotten reliable, rugged and cheap enough by the 1960s to make this gear very useful on the battlefield. It was also a lot safer to put a bunch of REMBASS sensors, rather than troops, out there where the enemy might suddenly appear. As a result of REMBASS, it became a lot more difficult for communist guerillas to move around. We tend to think of Vietnam as an unmitigated disaster, but if you read any of the books written (since the 1990s) by troops on the communist side, you get a sense that the American troops and all their technology were a pretty lethal combination.
After Vietnam, the sensors got smaller, more powerful and more reliable. The current system used in Iraq, REMBASS II, contains sensors that can detect sound (acoustic sensors), vibrations traveling through the ground (seismic), heat (thermal, or infrared) and metal (magnetic). Troops have to use some imagination in placing the sensors. You don't want them to be easily spotted by hostile troops, or just curious civilians (who might decide to take it home, along with the radio transmitter that is placed nearby and connected to the sensor via a wire.) The radio signal can be picked up 10-15 kilometers away (or much farther, using repeater stations.) The batteries in the sensors have to be changed every 30 days or so. You can also use an aircraft or UAV overhead to receive the signals from your sensors.
The big change with REMBASS II is the software in the laptop computer that is used to monitor each sensor network, as well as the sensors themselves. With decades of experience using all these sensors, it is possible to equip sensors with software that has information that enables the sensor to eliminate most false alarms. Thus the sensor fires off a message to the control system only when it has detected what it is programmed to detect (people or vehicles or only vehicles of a certain size of type and so on). The sensors can be used in combination to get more information. For example, by including a magnetic sensor, and you can detect people passing by who are carrying rifles (which are metal). There are also sensors that can, less reliably, detect explosives or chemical weapons being carried.
The U.S. Army Special Forces are particularly keen on REMBASS II. A lot of what Special Forces do has to do with stakeouts in remote areas. Special Forces troops believe fewer (people) is better when they go into hostile country. Sensors, carefully placed, allow a team of four to six Special Forces troops to monitor a huge area. In places like Afghanistan, this is essential if you are going to have a chance of covering the vast, unpopulated areas the bad guys can travel over. In Iraq, REMBASS is heavily used in protecting bases, and in staking out border areas that are being used by terrorists or anti-government fighters to smuggle themselves, and weapons, into the country.
REMBASS II is one of those items of military equipment that just don't attract much attention, but they make a big difference. All you see is a soldier working on a laptop, which is connected to what appears to be a radio. That's a REMBASS II control station, and that GI could easily be monitoring foot and vehicle traffic for miles around, 24/7. Spotting the enemy first has been a key military goal for thousands of years, and all REMBASS II does is use current technology to do it.