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