Here is my last 2 cents on this:
1)Why doesn't Apple do what they do best, develope and sell a product and make a profit
2) AT&T do what they do and sell the service and profit off that.
Apple shouldnt get any service plan revenue and ATT shouldnt get a cut of the iPhone profit..
Seems simple enough.
There's a couple of reasons, but one sticks out in my mind. The "iPhone experiment" is a very risky experiment for either Apple or AT&T to undertake alone. From the infrastructure part, Apple needs AT&T's network or this won't work. Period. However, Apple seems to have the intent of changing the way that cellular services works. AT&T is at least intrigued by Apple's way of thinking. So in order to distribute risk, they've essentially formed a joint venture for the iPhone, and as such profits are shared to some extent.