The pilot reported that he contacted a nearby FAA flight service station (FSS) to ask if any airplanes had taken off or landed at the snow-covered, unattended gravel airstrip that was his destination. The FSS specialist said airplanes had landed there earlier in the day, but he did not have information about the runway condition. The pilot overflew the airstrip about 500 feet above the ground, and it appeared to be hard packed, with signs of use. During the landing roll, the airplane's tires encountered soft slush, and the airplane nosed over, collapsing the right wing lift-strut. The pilot said there were no preaccident mechanical problems with the airplane, and that he was unaware that the airstrip was used primarily by ski-equipped airplanes during the winter.