Via Irving Reid, a great quote from Henrik Kniberg about how to tell when you're done:
So when a team member says that a story is Done and moves the story card into the Done column, a customer could run into the room at that moment and say "Great Lets go live now", and nobody in the team will say "no, but wait".
Ironically, Kniberg's post is called "Version Control for Multiple Agile Teams". Why ironic? Because I was talking yesterday to a professor in my department --- young guy, smart, already has a better research and publication record than I'll ever have. I brought up version control (as in, "Why do less than 10% of faculty and grad students use it?"); his reply was, "Well, we're just going to back everything up once a day."