Some bloggers have discovered the ultimate loophole. AT&T covers $250 of the cost of your cellphone if you sign a two-year contract -- basically subsidizing the cost of the phone. But you're also allowed to transfer your two-year contract to the iPhone without any penalty.
Here's the loophole. If you're going to buy an iPhone anyways, why not get the two-year AT&T service plan first with another phone, and then transfer the contract to your iPhone.