Typical subsidy on a 2 year contract is $150 discount off the full price. $75 for 1 year contracts.
So if the phone is $400, it should be $250 for a new subscriber taking a 2 year contract. That's they way it has been for all phones on AT&T, so unless they treat the iPhone differently, it this should be the case.
Some times there are mail in rebates but I don't expect there to be any on the iPhone.
The EFT (early termination fee) for breaking your 2 year contract is $175. Now that there are going to be pro-rated EFTs, I don't how that's different.
So if you plan to stay with AT&T for at least 2 years, it's worth taking the $150 subsidy. Even if you do leave early and pay the $175 EFT, you'd be paying $25 over what you would have paid for the full price of the phone. And of course now that there will be pro-rated EFTs it might make even more sense to do it.
What this means is if, for example you are with Verizon or T-Mobile, you'll be able to walk in to AT&T, "port" your number to AT&T, sign up for a 2 year contract, and get the iPhone for (presumably) $150 less than full price.
The port process would automatically cancel your service with the company you're already with. You'll get an early termination fee bill from them if you're still in contract or nothing if you're out of contract.