Originally Posted by solipsism
As you stated "iPhone", not iPad. not "all iDevices", not "any and all devices". If AT&T offers an option that is a pay for phone 3G device and get all other connected for free plan then you'll be good to go, but that doesn't seem likely. Comcast doesn't give me free internet and cable at my other homes simply because I can only be in one at a time. I have to pay to maintain service to each home.
Solipism, it is commonly accepted that Intenet service is a utility in the same sense that electricity, telephone, gas, water, or Cable TV are also utilities. AT&T, like any of these other utilities, provides you a service that should be device agnostic. There was a time when the Cable TV companies used to charge you an additional fee for each TV you had in your house, even though it didn't cost them any more money to provide service to those TVs. Thankfully, most companies have gotten away from those practices and just charge a standard fee for basic cable (although they can charge you per converter box). How would you feel if the electric company made you sign a contract for a certain amount of kWH per month but stipulated that the electricity they provided was only to be used to run your refrigerator, and if you wanted to hook up a lamp, you would have to sign up an additional contract for using that device?
As others have said countless times: data is data. How you choose to use it should be up to you. This is the basic tenet of net-neutrality. I see no difference in the argument that you should be able to use your data connection for any software service such as video streaming, voip, bittorrent, etc, and being able to choose what hardware device consumes the data that you pay for.
Consequently, if I pay AT&T for 5GB of data, I should be able to use those 5GB in the method of my choosing, be it for the iPhone, as well as my iPad, iPod, MacBook Pro or whatever through tethering.