While conducting an instrument approach, the pilot of a Piper PA-31T crossed the initial approach fix and descended to 2,300 feet, per the instrument approach procedure. Upon reaching the final approach fix, he then selected 1,200 feet in the altitude preselect and almost simultaneously visually acquired the runway and visual approach slope indicator. The pilot then "became distracted," and" forgot to extend the [landing] gear." He then fully extended the wing flaps, but the power setting remained above the "trigger point" for the landing gear warning horn. The airplane touched down with the landing gear retracted and was substantially damaged.