An AT&T employee says that any holidays for the last two weeks of September would be denied "due to an event blackout". According to him, the only times this has been done in the past has been when an iPhone was released:
Date: Fri, Jul 29, 2011 at 12:27 PM Subject: Iphone 5 release time
I'm an att employee and my boss just announced that vacation requests for the end of september will be denied due to an "event blackout". Historically the only time they've done this was for an iphone release. So we're looking at the last two weeks of september.
This coincides with previous rumours, so it makes sense. Like always, take rumours with a spoon of Maldon salt.