Last year I bought a 3GS but I had to first become an authorized user. The main user on the account has to do this but I think it can be done over the internet. It's best to check on the website to see how to become an authorized user. If you're not and can't figure it out online, then go into an ATT store and ask them to make you one. I believe all I needed was the Social Security Number of the main user. After that I had no problem. I went in to preorder this year and I had no problem. Good luck!
It's unlikely they will actually activate in the store. That being said they will validate your account and that you are an authorized user. You may need the primary account holder's info as well. They did activations in the store in '07 and it was a cluster ****. Ever since they've let you pay, walk out with the phone in box, and activate it yourself.
To get the pricing on the phone with a contract, you need the main account holder to be with you and have the contract be valid for an update. Otherwise, you will be paying the full price for a phone.