I believe in all years past preorders go live the day after they announce and usually always at 2 or 3:00am as far as I remember. I believe the 4 or 4s strayed away fom this and did midnight possibly.
But I know for sure at least 3 out of the 4iPhones I had to set my alarm for 2:00am to be there when preorders went live... Within an hour though the servers strt to crash though and a *****torm for hours follows trying to place your order.
Oh, and preorders won't go live on the 12th, most likely the day after/13th between 12:00-3:00am