May 2009: The mystery berry is no longer a mystery, a kind person in Bellefonte, PA, found what sure looks to be this plant.
Persicaria perfoliata (formerly Polygonum perfoliatum) http://www.na.fs.fed.us/spfo/pubs/pest_al/mm/pa_mam.pdf an invasive species!
Apparently it out-competes many native plants, grows to 8 meters in height and can completely blanket large areas of low growing brush.