Launching with iOS 8.1.
AT&T Offers Walkthrough Videos for iPhone 3G Buying
Existing AT&T Customer Video
This covers the different scenarios if you are an existing AT&T customers. AT&T recommends you back up your iPhone with iTunes prior to buying the iPhone 3G. Additionally, voicemail will be deleted when you upgrade, so you should be sure to catch up on your voicemail prior to upgrading. Once you upgrade to the iPhone 3G, you can give your original iPhone to someone else who can activate it through iTunes and sign up for new service.
New AT&T Customer Video
This video is for customers activating new service at AT&T. These individuals can visit AT&T stores prior to July 11th to undergo a credit check to save time on the actual launch day. AT&T recommends you back up your phone data prior to July 11th. You will need a photo ID and social security number to start new service. If you are interested in porting your phone number from your existing carrier, you will need: your account number, account holder name, billing address, social security number and PIN or password. Most of this information is on your bill from your current carrier.
Apple has not yet announced what time their retail stores will open on July 11th. AT&T announced plans and pricing for the iPhone 3G earlier today.