I too have set up a mailbox. I chose as my site the empty lot next to the Potrero Hill bar, The Bottom of the Hill.

The address of the box is
1229 17th St,
San Francisco, CA 94107

Backstory:
I found someone giving away two mailboxes on craigslist. Seemed perfect for this task. The boxes are pretty decent. I had to replace the lock on the one I put up, but it wasn't too hard. I'm not totally sure the locks will keep a determined person out of the box, but hey, its worth a try.

I secured the box as well as I could. I attached it to the fence with hose clamps which seem pretty damn solid. You'd have to be on the other side of the fence if you wanted to take it down without destroying it. I've included some pictures and will check up often to see if mail arrives. (I will also send something just to see if the post office will deliver there.)

I also realize that I skipped a step in the proof, which is to actually receive a message at the box, but how am I going to do that if no one knows it is there? I figured if Sam could get away with it, I could too. Perhaps I'm being presumptuous. Also, I have the added security of knowing that I have a second box if the first one gets destroyed. So one way or another, I'm going to get it done.