According to an email sent by Nuance to its developers, iOS 7 GA (General Availability) will be released on September 10. It asked developers using its API to conduct testing to ensure that their services work on iOS 7 even though its SpeechKit 1.4.5 is pre-qualified to work properly on iOS 7. Nuance is the voice recognition focused company which has also worked on Siri, Apple's intelligent voice assistant. This information was first reported by Owen Williams.
It's possible that Nuance could have made an error and that it intended to inform developers that the OS' Gold Master version was being released on September 10. It's rare that any entity outside Apple gets detailed information about the iOS release plans. However Nuance is closely associated with Apple, so it's possible that the company is aware of the date.
Typically, Apple seeds the Gold Master or final developer version of iOS on the day it announces the iPhones. This enables them to test their apps with the final build of the OS before it's seeded to the public. It then releases the OS before the iPhone hits the market.
If the date of the general availability of the OS is indeed September 10, then Apple would probably seed the Gold Master release of iOS 7 to developers within a few days.
It's worth pointing out that the September 10 date for Apple's iPhone event has been derived from rumours and the company has not sent invitations or announced an event officially, yet, though it typically sends out official invitations only a week before the actual event.It's also not clear if the OS would only hit iPhones, with an iPad release scheduled for a later date, as indicated by some reports.