THE PLT REPORTED THAT THE ENGINE BEGAN TO RUN ROUGH & LOSE POWER AS THE AIRPLANE CLIMBED THROUGH 9,000 FT MSL. SHE THEN SWITCHED FUEL TANKS AND MOVED THE MIXTURE TO FULL RICH, BUT THE ENGINE CONTINUED TO LOSE POWER. CARB HEAT WAS NOT USED AT ANY TIME. A FORCED LDG WAS SUBSEQUENTLY MADE IN A FIELD WHERE THE AIRPLANE COLLIDED WITH A UTILITY POLE AND A DITCH. POST-CRASH EXAM OF THE ENGINE REVEALED NO EVIDENCE OF PREEXISTING MECHANICAL FAILURE OR MALFUNCTION. EXAMINATION OF AN ICING PROBABILITY CHART REVEALED THAT THE REPORTED WX CONDITIONS IN THE AREA WERE FAVORABLE FOR THE FORMATION OF MODERATE CARBURETOR ICING AT CRUISE POWER. THE CESSNA 172M OWNER'S MANUAL NOTES THAT A GRADUAL LOSS IN RPM AND EVENTUAL ENGINE ROUGHNESS MAY RESULT FROM THE FORMATION OF CARB ICE, AND PRESCRIBES THE USE OF CARB HEAT TO CLEAR THE ICE.