No third party apps on the iPhone:
So why won't Apple make the iPhone an innovative environment?
It's all about the money honey.
Stop believing naively in a benign Apple. The carriers hate an uncontrolled phone environment. Ergo, Apple will need to control the phone to make greedy money. Because, you guessed it Apple makes more money working with the carriers than fighting them. The iPhone innovates with the help of the carrier, think visual voice mail. OTOH, a full featured OS X running phone would make SIP/VoIP trivial.
An open platform will lead to SIP / VoIP, the carriers need a couple of years to control the VoIP market through usable offerings of their own, and they need to cripple better choices till then. It'd be really bad timing for Apple to deliver a full feature phone this early. Apple is at the mercy of the carriers who subsidize every popular phone in the US market. A popular phone with carrier subsidy will end up selling more music for Apple than any other medium.
A powerful computing device in the hands of the many will prove to be too disruptive a force to established businesses, including Apple. Heck you'd have people cracking DRMed music for the iPhone, having OGG and DivX running directly on the iPhone, and in short ringing a death knell to the iTunes walled market. Apple clearly intends to make money off the iPhone, and it's not merely by selling the hardware.
Of course there are other reasons too,
The UI is completely unique in the industry today, Apple will want to gain maximum mileage from it, and understand how the UI can be used optimally before opening it up to 3rd parties in a controlled manner. In a couple of years if the UI proves to be successful there will be innumerable clones on the market, at which time the Apple UI libraries will be infinitely superior and offer greater control. Hence possibly mad profit.
The processor maker, Intel has an interest in ensuring that the iPhone does not kill the handheld / tablet / micro-notebook market for a while. It's also clear that too much computing will kill the battery in short order increasing customer dissatisfaction and leading average joe to think twice about buying the phone.
Of course all of this is assuming that Apple has not as usual made bloat ware, where loading any extra app on the phone would slow it to a crawl and make it unusable. i.e. make you miss your calls because the color wheel of patience is busy spinning.