Well here is the scoop. Im assuming if you have a regular account with ATT you will have no problems doing this. You will simply pop in your sim, open itunes, it will show your phone number. You enter in your zip code and last 4 digits of your social, it runs a quick check and you are good to go.
BUT......Im on a gophone plan. You CANNOT re-activate through iTunes by just swapping sims. After it runs its check, you will get a message that says "we're sorry, ATT has determined your current account cannot be used on the iPhone."
A call to Apple was no help. They transferred me to ATT and after a quick stint on hold the CSR said "oh sorry, you cant do this with an iPhone." Irregardless of the fact that I already have a working and exsisting gophone account, your ONLY option is to run through the new account set up again and set up a NEW gophone plan with an new number. And all your current credit will be lost, cant even transfer that.
So after some serious ranting and raving, I set out to use an alternative method to activate. Was a bit tricky and made me a bit nervous, but after a few hours of digging around I found how activate AND get the old sim from my 4gb phone to give me phone service on the new 8gb.
Its really ridiculous to have to do all this though. If you have an exsisting account, gophone even, why make this so difficult??
Apple told me that ATT is solely responsible for the activation, they have nothing to do with it and cannot help you bypass it.