Lets put it in simple form so everyone can understand and quit crying.
When you bought the iPhone 3G you paid for a 1/3 of the phone and agreed to pay AT&T for the rest of the phone. That price does not change at all--It is $400.
So if you plan small monthly plan it takes you longer to pay it back---Say 20 months in some circumstances.
Now if you pay more than it takes less time to pay off the phone.
This is how it works everywhere people, not just AT&T.
The arbitrary dates are not arbitrary but very calculated by AT&T. They simple determine the exact day you are done paying for the iPhone 3G, thats it.
The main problem is there are many plans out there and no one is truly a like on these forums. Sorry thats just the truth. People come on a say I pay $400 dollars and I want a new phone!! Well ok lets punch it into if you have paid off your subsidized iPhone. Well I have a family plan with 8 phones and the main phone only costs $75 a month. Well all they care about is the main phone and that will take 20 months to pay of the phone.