The pilot was taxiing an airplane across an open lighted ramp area at night. He observed a tug closing from his left side and when the tug did not give way to the airplane, he initiated braking. The tug struck the airplane on the nose, inflicting substantial damage. The tug driver reported that he did not observe the airplane in time to prevent the collision. No obstructions to visibility were present for the pilot or tug driver. Airport rules required vehicles to yield the right-of-way to airplanes.