Is it possible to divine the iOS 5 release date by looking at beta expiration dates?
While this is a possibility, I don't think the beta expiration dates really mean anything in terms of pinning down an exact date when software will be released. There are still numerous betas that will be released (at least 1 to 2 more… and Apple is currently tracking a number of major issues this late in the development cycle) and their expiration dates will be moved accordingly -- potentially to even after the iOS 5 release date.
A prior example of this is the Windows 7 RC -- which expired on June 1st, 2010. Windows 7 was officially released on October 22, 2009.
Expiration dates for iOS 5 betas
iOS 5 beta 1: August 4th, 2011
iOS 5 beta 2: August 4th, 2011
iOS 5 beta 3: August 18th, 2011
iOS 5 beta 4: August 31st, 2011
iOS 5 beta 5: September 14th, 2011
iOS 5 beta 6: September 29th, 2011
As you can see, the beta expiration dates seem to follow the two week release cycle of the betas. New beta released? Pushed the expiration date back a few weeks.
This means we'll probably see iOS 5 beta 7 expire around October 13th or 14th and iOS 5 beta 8 (if needed) expire around the end of October. Does this mean iOS 5 is going to be released in November?
Secondly, even if you knew which beta was the last one before the GM, they wouldn't time the expiration date to force you to upgrade immediately. If they were going to do that, then why wouldn't they make beta 5 expire right after beta 6 came out?
Beta 5 expires almost a full month after beta 6 came out, so if anything, the final beta will probably expire roughly a month after the planned release date. But again, we have no way of knowing which beta is the last one before a gold release.