My husband and I were driving on a hilly river road in our small town in late August when I spied a beautiful flower garden in a sunny backyard and exclaimed quite eloquently, "Holy crap! What's that?!" There, in a long bed lined neatly with field stone, grew a beautiful plant with long pink blooms that towered over the rest of the garden at roughly 10 ft tall! I'd never seen it before and I just had to know what it was.
We drove home and I began my GPS'ing (Google plant sleuthing). I've become quite a whiz at this, feverishly typing descriptive phrases to identify my latest mystery plant.
Finally, after searching in spurts for several days, I found what I was looking for. Polygonum orientale, or it's much more romantic common name, 'Kiss-me-over-the-garden-gate'.

This link provides some good info on this charming annual. I wonder if the owners would notice me creeping up into their yard to gather some seeds...