Many have assumed that iOS 7 would be released on September 10, a date that came to public attention after an Apple event was leaked. While seemingly supported by anecdotal evidence, the date has apparently been confirmed by an email that Nuance sent out to developers, in which the company states that the iOS 7 general availability release is scheduled for September 10.
The letter was made known by developer Owen Williams, who posted a screenshot of the email he received from Nuance - the business behind Siri - that started off with a confirmation of the release's general availability. While such a statement should be taken with a dose of salt, Nuance is just about as authoritative of a source as it can get in this sort of matter.
Said the letter in part: "As you are probably aware, iOS 7 GA will be released on September 10th. The NDEV Program has pre-qualified that SpeechKit 1.4.5 works properly on iOS 7, but we encourage you to conduct your own testing to ensure that went the upgrade happens, your Nuance speech services continue to work seamlessly..."
Still, the date has not been officially confirmed, and the possibility always exists that Nuance is not basing its email off of an official confirmation from Apple, but instead from the same rumors that others are hearing. Adding to the speculation, the email was previously available on Nuance's servers, but since Williams' screenshot was posted it has been pulled.