The pilot stated that he returned to land at the grass airstrip after a glider tow, and his approach and touchdown were normal. During the landing roll the airplane encountered a soft wet portion of the runway, '...the tail came up and [the airplane] nosed over.' The pilot reported that there was a 10 knot quartering tail wind while landing. The FAA Inspector stated that the grass runway was well maintained and mowed, and the wet section was not visually evident prior to the accident.