It looks like Apple is set to release the final version of iOS 5 to its overseas assemblers during the week of September 23rd, prepping it for a mid-October release alongside the iPhone 5.
The overseas assemblers need to get the new software in order to preload it onto new phones before they're shipped, so the timing lines up pretty well. Apple is expected to announce the iPhone 5 within the next couple of weeks, and then release it next month. When the iPhone 5 is released, iOS 5 should be available to download for owners of older iPhones as well. And the circle of life continues.