Edited 7/21/09. The nice folks over at Garden Web have been kind enough to help me identify this as an ironweed (vernonia gigantea, to be exact.)
I'm no Jack but I think I've got myself a beanstalk growing off the back side of my deck. This baby's been growing at the rate of about two feet per week and has these little berry like blossoms that I swear are going to bloom sometime before I die. If I weren't so sure of this, I'd just cut the monster down. It really isn't attractive at this stage, but curiosity has gotten the best of me. Any ideas?