(Ed. 11/07): Just found that it's milkweed. But you knew that, right?)
Yesterday, I took my usual weekly drive to Bedford, and when I was coming back, I saw what has happened to them since the weather has warmed up and we actually had an Autumn. I couldn't stop and take a picture then, so today when I was out running errands, I took a little trip to take some pictures.
Aren't these plants amazing?