Apple may release iOS 7 to the public the same day as its accompanying iPhone event on Tuesday, September 10th. In the past, Apple has released the final GM build of the next iOS to developers on the day of the event and then waited about a week to push the update out to everyone. This year could be different.
An email sent out from Nuance to developers in its voice recognition program suggests that iOS 7 will indeed drop on the 10th. And the fact that Nuance powers Siri lends credence to the date.