updated 04:33 pm EDT, Tue August 21, 2012
Blackout just little over one week
Verizon Wireless is blocking off vacations for all of its workers between September 21st and the 30th, according to a source in the company. Because carriers rarely institute this kind of ban, it's believed that the timeframe indicates when Apple will launch the next-generation iPhone. Recent rumors have, in fact, pointed to the 21st explicitly.
As for an announcement, a wider set of reports appear to agree that Apple will hold a press event on September 12th; preorders may even begin the same day. AT&T has also previously been reported as arranging a vacation blackout, but over a longer period of time, making it hard to pinpoint a launch date.
The period immediately after any iPhone launch is often extremely demanding on US carriers. Each company has to not only handle intense online and retail sales demand, but activations for anyone choosing their service, regardless of where a person bought their phone. In some past iPhone launches carriers have been completely swamped, forcing people to wait for hours before their phones would work. In recent years the situation has smoothed out.