It is pretty much consensus in the Apple rumor mill that Apple will unveil the iPhone 5 in September. When the iPhone 5 is actually shipping has not been discussed much yet. UK's gadget magazine T3 picked up a rumor that the iPhone 5 launches on November 21 in the UK. The information comes from a representative of phone retailer Phones4U. It is hard to say how much time will pass between the iPhone 5 (or is it just an iPhone 4S) announcement and the release. If this iPhone 5 UK release date would also be true for the US, it would put the iPhone 5 launch in the crazy Black Friday week (Black Friday 2011 is Nov. 25).
My guess is though that Apple is not waiting that long with the iPhone 5 release. Next month they will unveil iOS 5 at the WWDC 2011. Developers will then have all summer to prepare their apps for iOS 5. So there is no need for Apple to unveil the iPhone 5 and than wait for developers to adjust their apps. I think Apple will start the sale within a week or two after the September announcement.