DARPA wants to use the BigDog to carry supplies for soldiers over terrain inaccessible to traditional vehicles.
Sense: to traverse said terrain, BigDog will need an extremely good and thorough representation of the terrain to work with. To wit, from the BigDog wikipedia entry: "The legs contain a variety of sensors, including joint position and ground contact. BigDog also features a laser gyroscope and a stereo vision system."
Plan: An autonomous BigDog would require full-blown path planning. As far as I know, global paths are determined by accompanying personnel, but an onboard computer handles navigation and control around local obstacles.
Act: The quadruped nature of the BigDog's movement is fairly obvious from the image. Still, since its purpose is to march forth and be unstoppable, its balancing systems are very impressive. YouTube video of BigDog can attest to this, which is accomplished by the onboard computer and great mechanical engineering.