I can't believe we're 4 days from the launch now, and even us forum people, much less the general public, or even the employees in the AT&T and Apple stores don't know exactly how these phones are going to be activated.
As far as the Apple store, if those employees were really going to be signing people up for AT&T plans, and handling issues and questions regarding migrating from some other AT&T plan to an iPhone plan, it seems like their training would have had to have started much earlier than this week. It would probably take weeks of training, or learning under someone else.
I see two options for the Apple store at this point:
1) they just sell you the phone and unbrick it, taking your credit card number while making you sign an agreement to activate it on AT&T within x number of days or the card gets charged a large fee
2) they have several AT&T reps in each Apple store at launch, with at least one permanent AT&T terminal somewhere in the store, and probably other reps with laptops for launch day to get people signed up