I will suggest this to you and the other guy in your boat....
Go the apple store right now, and select to purchase an iphone 4s. Select ATT and hit continue.
Then select the option "Add a new line to my existing account"
it will ask for info to check eligibility of adding a line to your account.... you will know from there what your options are. If you could report back and let me know what it says, Im interested!
after checking eligibility, it takes you to a page where you can "join an existing familytalk plan" or select a mobile share plan. after choosing "join an existing familytalk plan," it will take you to a page where you can select a data plan (300mb, 3gb, or 5gb on ATT). It will then take you to a page where you can choose your messaging plan. Then, it will go to a page where you can choose "give me a new number" or "transfer my current number." the last page asks you to "confirm your address." then, you are able to add to cart and i assume order.