Here's something from a poster on AT&T forums:
Calm down, people.
I know there are a lot of folks frothing at the mouth over this and can't fathom why there's an apparent delay in iPhone 6+ shipping. Here's what happened, as I understand it:
Pre-sales were supposed to go live, across the board, at 12:01 AM PST. People loaded up the Apple site or their carrier site of choice (or both). Some folks are still unaware of this, but a technical error allowed people to order at least 15 minutes early from AT&T (I was one of them - I got my order in at around 11:54 PM and still got an initial ship date of 10/02). A lot of people took advantage of this, leading to a large number of stock being spoken for before the "official" presale time. This is why people who ordered minutes after midnight from AT&T were getting mid-October ship dates.
12:01 rolls around and Apple was still having problems, which caused a significant number of people to switch over to the carrier sites to make their orders. The huge influx of orders which otherwise would have been placed at Apple caused AT&T's projected ship dates to push back two months. Most people were shocked upon seeing these ship dates and cancelled their orders, and instead ordered from Apple (or somewhere else), hoping to get their phone on launch day.
Here's the kicker, though: It doesn't matter where you order from. They're all coming from the same source (Apple), and they will be shipped in the order in which they were received. If your order is processing or preparing for shipment, DO NOT CANCEL IT. If you cancel, you're essentially losing your place in line and you're only going to get it later than you thought you were in the first place. Don't take your aggression out on AT&T's employees. It's not their fault, and they're trying to deal with this fiasco as best they can. There are a ton of cancelled orders due to impatient people, and those will have to be cleared from their system. Once that happens, it will become apparent where you stand, and even more apparent when the tracking numbers go out next week.
From what I gather, if you ordered within the first few hours, you'll likely get your phone on release day. But even if you don't, so what? I know it's the "new hotness," but in reality, it brings little new to the table. If you currently have an iPhone, it can already do everything the 6 can do. There's been a lot of outrage over this ordeal, and most of it sounds exactly like the whining of a petulant child who didn't get the toy they wanted when they wanted it.
Put things in perspective, chill out, and give AT&T (and everyone else) a few days for the smoke to clear. You'll get your phone.
May not be true or the exact cause but it makes me feel better.