The iPhone 5 will be announced in mid to late September, will begin production in early september and be released in early October. The reasoning behind this is the white iPhone 4. Apple is a genius in marketing and timing.
The iPhone 4 in white should be available sometime very soon and the ignorant people (being nice here) that have been holding off for it will all flock to it because it's the newest and coolest apple product, even though it isn't new at all as far as technology, speed, and productivity go. So Apple will stagger the release of the iPhone 5 in order for people to get over the white iPhone and be anxious for the iPhone 5, wanting an early upgrade. It will build up even more hype than currently present, ultimately leading to even more profit for Apple and more devices sold.
In these times when Apple wants to control the smartphone market instead of Android, they are earning their place very cleverly. With the release of the iPhone 4 on Verizon, Apple became even stronger of a competitor in the smartphone market in the US. With the release of their Q2 earnings yesterday, Apple reported selling 18.65 million iPhones..... That's over twice as many iPhones as they sold during last year's Q2. Apple will be looking to keep that number strong with the release of a white iPhone 4 in Quarter 3. So instead of releasing the iPhone 5 in that same quarter, Apple will look to grow in their market share even more and hold off the iPhone 5 until October.
But then again, this is all just logical speculation