Existing AT&T iPhone Customers
You're immediately eligible to upgrade to iPhone 3G at the $199 upgrade price for the 8GB model, plus an $18 upgrade fee. Alternatively, you can upgrade to the 16GB model for $299 plus the $18 upgrade fee.
Before going to an AT&T or Apple Retail store to upgrade, you'll want to sync your current iPhone's contacts, calendar, photos, emails, music and other media files to iTunes as you normally would. Although your new iPhone 3G will be activated in the store, you'll need to bring it back home and then sync it to iTunes in order to populate it with your contacts and data from your old iPhone.
You'll also want to check any voicemails you have on your original iPhone before upgrading to iPhone 3G, as your voicemails will not be carried over and will be deleted.
You'll then have to decide what accessories you want to purchase with your new iPhone 3G, what calling plan you'd like to sign up for, and whether you'd like to give your old iPhone to a family member of friend.
Existing AT&T Customers who don't currently have an iPhone
If you're an existing AT&T customer who has a mobile phone other than an iPhone, you'll first need to determine whether you are eligible for upgrade pricing. To do this, log onto www.att.com/iphone/ and select "Log In" at far right hand side of the orange bar. Then type in your current mobile phone number and your password. If you're not yet registered for an AT&T online account, you can do so from the login page. Once logged into your account, select the "Upgrade Phone" link from the far right-hand side of the page. That page will tell you whether any of the existing mobile phones on your account are currently eligible for upgrade pricing.
If you're currently eligible for upgrade pricing, you can upgrade to iPhone 3G at the cost of $199 for the 8GB or $299 for the 16GB model, plus an $18 upgrade fee.
If you're not currently eligible for upgrade pricing, you can upgrade to an 8GB iPhone 3G for $399 or the 16GB model for $499.
Customers new to AT&T
If you're switching to AT&T to obtain an iPhone 3G, the first thing to remember is that there is no benefit to switching your service prior to iPhone 3G launch day on July 11th.
Customers new to AT&T can purchase iPhone 3G for $199 for the 8GB model or $299 for the 16GB model with a new 2-year contract and $36 activation fee.
Before arriving at an AT&T or Apple store, be sure to review the iPhone 3G rate plans so you know which one you want to select on July 11th.
Second, you can get a Credit Check out of the way by visiting the store where you plan to purchase your new iPhone 3G ahead of July 11th.
Third, backup any data that's on your current mobile phone that you want to put on your iPhone 3G. Since the iPhone 3G syncs with applications on your Mac or PC, getting your current phones data to a computer is a good move.
When you're ready to buy iPhone 3G on or after July 11th, you'll need to bring AT&T some additional information. Since you'll be opening a new account, AT&T will request that you provide a photo ID and your social security number to perform a credit check. As mentioned above, this can be taken care of ahead of July 11th. Depending on your credit, AT&T may possibly ask for a deposit.
Next, AT&T will be ready to port your existing phone number (number porting FAQ and checklist) to your new iPhone 3G. For AT&T to do this, however, they'll need your existing account information as it is on record with your existing wireless provider. This information is usually contained on your existing phone bill, so bringing a copy of your existing phone bill to an Apple or AT&T store will speed up this process. Specifically, AT&T will need your existing 1) Account Number 2) Name 3) Billing Address 4) Social Security Number of Tax ID and 5) Your existing PIN or Password (if you have one).
If all the information matches properly, AT&T promises you'll be able to make calls on your new iPhone 3G "within minutes."
If AT&T tells you your out-of-area mobile number is ineligible for porting, try the steps taken by AppleInsider's Publisher last June when he successfully ported his 'ineligible' mobile number to AT&T and iPhone.
Earlier today, AppleInsider detailed the increased costs associated with iPhone 3G after outlining AT&T's standard service plans for the new touch-screen handset.