The Apple Store is back online for iPhone 6s and iPhone 6s Plus preorders, though a variety of quirks have affected some users. Despite such issues, many people have successfully booked their preorder for delivery or pick up on September 25th, 2015.
Problems reaching TMO include some credit cards (Visa, in particular) resulting in kicking the user out of the odering process; the Confirm Store Reservation button not working; some people not getting the Web version of the store up and running; and my favorite—AT&T's eligibility payment system scheduled maintenance for midnight to 2:00 AM PDT, the first two hours of eligibility, as shown in the screenshot below:
Slick move, Ma Bell.
Still, preorders are going through for many customers, as shown below.
One Lucky Preorder's Confirmation
As of this writing, all models are still showing as available for delivery on September 25th, 2015.