Here’s a thought: the iPhone and iPad are testing grounds for each other.
Steve Jobs said that Apple began building a touch device by first working on the iPad. But they set it aside to build the iPhone first instead. The iPad was the first idea, the iPhone was the first product shipped. The technology and operating system of the iPhone was then used as the foundation to build and ship the iPad.
The iPad was the first device with the A4 chip. Now the iPhone has it as well. The iPad now has the A5, and that is likely coming to the next iPhone.
The iPhone was the first with a front-facing camera and a Retina Display. The iPad has the former and it will soon have the latter.
The iPad has 3G data connectivity without a carrier contract. The iPhone doesn’t (yet).
The two devices keep leapfrogging each other. They swerve in and out of each other’s development cycles. Each one gets its own and different type of technology and then passes it on to the other. Sometimes the iPhone gets it before the iPad, and sometimes the iPad gets it before the iPhone.