According to iMore.com, we may see iPhone 5 pre-orders as early as 12 September with an official launch 9 days later on 21 September. Not sure if this is a sure thing or not but, historically, this has been a good source of intel on all things i-whatever.
http://www.imore.com/iphone-5-pe-orders-planned-september-12-second-release-wave-first-week-october