B737-400 Captain reports a stall warning as the First Officer rotates on takeoff and remains activated throughout the clean up and initial climb. The crew elects to return for an overweight landing and the stall warning disappears when flaps 30 are selected. A flap position sensor was found to be the culprit.

Narrative

At rotation the stall warning (stick shaker) activated. It remained activated as we continued with the climb; acceleration and reconfiguration of the aircraft. The after take-off checklist was completed and we continued to 4;000 FT; where we leveled off. As we continued to accelerate; the stall warning/stick shaker remained active up to and including 240 KTS indicated airspeed. We advised the Tower we wished to return and land; but needed delay vectors to run all necessary QRH procedures. Once these QRH procedures were complete; we received vectors back. During the reconfiguration for decent and landing; the stall warning/stick shaker became intermittent. After the aircraft was in full landing configuration (flaps 30) the stall warning/stick shaker stopped and did not reoccur. An overweight Landing was completed softly; safely and without incident. Aircraft returned to the gate and log book entries were made. Maintenance Control was contacted and maintenance was performed on the aircraft overnight.We flew the aircraft the next day without incident. The stall warning sensor in the right flap caused this incident according to the log book write up made by Maintenance.