C172 pilot reports inability to climb after liftoff and after a porpoised landing; rejects using the entire 5;000 FT runway.

Narrative

Aircraft was flown to an airport under VFR conditions for fueling. Aircraft was fueled and immediately turned for the return flight. Pre-departure checklist/run-up was omitted. Aircraft accelerated as normal and rotation initiated at Vr. Aircraft climbed approximately 10 to 15 FT and climb was completely arrested. Autopilot was verified off and power setting and mixture rechecked. Due to inability to climb throttle was pulled to idle and aircraft began to porpoise. Throttle was completely opened in an attempt to regain flight and arrest porpoising. Inability to climb still occurred; at which time throttle was completely closed and a landing using almost the entire length of the 5;000 FT runway was achieved. After taxiing back it was determined that elevator trim was not set to the takeoff setting; and it had been left at the setting used on the previous landing. The previous landing had been accomplished with 20 degrees of wing flaps deployed and a more than usual amount of nose down trim due to thermals (95+ degree local temperature). No visible damage to aircraft; and aircraft performed as expected/normal on return flight. Aircraft did not contact any object/structure nor did ever leave the runway environment other than while briefly airborne over the runway during the failed takeoff roll.