I was told by AT&T GoPhone support that the 3G iPhone will be available as a Prepaid GoPhone. Rumors are now saying it won't be available as a GoPhone. Again this isn't set in stone anywhere.
"How does contract sign-up work when a customer is in an Apple store?"
"You will need to speak with Apple for more details on their retail operations, but the activation process will take place in the store." That's the official line, but we're also hearing that customers in Apple stores may take their purchase home with them without being physically activated, in which case the activation process is completed later through iTunes. Either way, though, they're not letting you out the door without signing on the dotted line.
In AT&T stores, the process should be pretty similar, except that customers will be obligated to physically activate before they leave. The stores will be receiving "tether cords" to make that happen. Just a warning, AT&T: whatever software you're activating these puppies with is going to be reverse-engineered in no time, we'd wager."
Stay tuned as all of this is subjected to change.