Techcrunch has it on good authority from a Verizon employee that the next iPhone will be launching on September 21 in at least the US, as all Verizon employees have a vacation blackout extending from the 21st to the 30th of the month. These blackouts are put in place whenever there is a massive product release, and the corporate team believes there will be enough demand that all hands will be needed in order to keep the process even remotely smooth.
This date lines up perfectly with iMore‘s rumored release date, as well as the various European carriers that have come out publicly, be it intentionally or not, and said that the next iPhone is going to be shipping in mid-to-late September.
These vacation blackouts are often the final pieces to the timeline puzzle; we will likely hear that AT&T and Sprint have instituted the same measures for the same (or at least mostly the same) dates. The penultimate piece will fall when the Apple Retail stores have posted similar blackout dates, and the final piece will be revealed on September 12th, with Tim Cook on stage.