BGR confirmed with its reliable source, and WPCentral claims to have seen internal documentation which specifically cites the 21st as “an important day for the upcoming Nokia flagship phone.” WPCentral later explains that their source has some seniority, and was on-target with the updates for the Mango release of Windows Phone.
As with any rumor, it is still a good idea take it with a bit of reservation, and since Windows Phone 8 was just released to manufacturers, it is perfectly understandable to think this date is optimistic. It gives Nokia and AT&T only a month to round out final testing of the device and its performance on AT&T’s network. This could also be a pre-order date with an early November delivery.
There is also the idea that this is an attempt to get the new Lumia in front of people as quickly as possible since the new iPhone is going to be on back-order for a while. That is certainly a sound decision, if Windows Phone 8, and the Lumia 920 are ready. That risk assessment has obviously been weighed, since a half-baked launch would be devastating to Nokia, and not shine too well on Microsoft and AT&T.
If it is ready though, why wait right? That means all the sooner we can get our hands on one!
sources: WPCentral, BGR