I believe it is three reasons -
- Still strong sales of the 4.
- Overheating problems of the A5
- Moving it to Sept could also mean this becomes their new schedule to announce new iPhones instead of June.
Will get them to prep for the Holiday season. iPod sales are on the decline. Most people just pass on their old iPhones to their children as iPod touches. MP3/PMP player market is becoming overkill and too niche. Running its course with the iPod touch. Apple pretty much killed off upgrading their iPod classics because of slow sales. And iCloud is not going to help them revive it.
I am also thinking if Apple is waiting on what Ice Cream Sandwich might look like. Google is going to unify phones and tablets. This can eliminate some of the fragmentation and give Android users more timely updates. By the time iPhone 5 comes out with the same A5 chip found in an iPad running at 1.2GHz dualcore, the HTC Vigor gets announced and is running at 1.5GHz dual core processor, has the new Ice Cream Sandwich, and has the first 720p display for a phone. You can wait for the iPhone 5 all you like, but newer technology waits for noone.