I just realized that by pushing back the iPhone to October that the iPhone will likely launch alongside new iPods as well. iPods always come out in October so they'll probably talk about iPods at the little conference they are going to have.
Anyway, I think the 21st is the more likely date since the Best Buy document shows that. I know when I worked for Best Buy our opening managers showed up at 7 and we opened at 10. So to have them there at 6 makes sense so that a 3rd party can set up that display.