Prior to departure on a positioning flight, the pilot of the helicopter checked the weather and determined that visual meteorological conditions existed at both his departure and destination airports. However, while in cruise flight at 400 feet above ground level while following a highway, the ceiling rapidly became lower, and the pilot encountered instrument meteorological conditions. Moments later at an airspeed of 65 knots, marker balls appeared which indicated that power lines were directly in front of him. He lowered the collective and entered a right diving turn in order to avoid the wires but one of his main rotor blades made contact with a power line. The rotor speed remained within limits but the helicopter began to vibrate. As a result, the pilot decided to land in a nearby field. Post accident examination of the helicopter revealed that the main rotor blade that had made contact with the power line was substantially damaged.