Pre-orders and previews begin April 10, launches April 24.
Nuance Developer Email Claims iOS 7 Will Launch September 10
It's unclear how Nuance would get this information, but it's unlikely that a company outside of Apple would have detailed information about Apple's iOS release plans.
Historically, Apple has released iOS Golden Master versions to developers on the same day as the announcement of new iPhones, giving developers time to test their apps with the final version of the new iOS before its release it to the general public a couple of days before the launch of the new iPhone.
Apple will reportedly announce the next-generation iPhone on September 10 and will likely unveil both the iPhone 5S and the lower-cost iPhone 5C. Both devices are rumored to launch in select markets on September 20.