T-Mobile has apparently blocked employee vacations between September 20 and September 22, which would coincide nicely with the expected release of the iPhone 5's successor. Apple will be hosting a special iPhone-related media event on September 10, and typically new models ship about a week later, or in this case, September 20.
News of the vacation blackout dates comes via the T-Mobile related news site TmoNews. The site doesn't say who its sources are, but the timing does fit with current iPhone speculation.
Apple hasn't officially announced its September 10 media event yet, although both AllThingsD and The Loop have said the date is set. Both have a reputation for nailing Apple's event dates, so this one is pretty much good to go and is just waiting on the invitations.
It's also possible that the details about T-Mobile's employee blackout dates isn't legit, but with the number of leaks surrounding the iPhone event, the timing does fit well.