Okay, after numerous conversations with AT&T, I finally feel satisfied that the information that everyone here seems to agree on is the correct information (as I suspected). I've been a long time reader of the forums, but finally decided to register because of my own frustration with this whole process that everyone else seems to also be experiencing.
I am doing an upgrade swap but because of the pre-order snafu I ended up with just a reservation at the Apple store. My concern was not being able to purchase the iPhone 4 at the reduced price because of all the issues that have been going on. I was told by AT&T 611 customer service the first time around that Apple could not do upgrade swap. I then went by the AT&T store and spoke with the manager, who said it would not be an issue, that Apple just had to contact AT&T during the activation process. This was last week.
Today I called 611 because I've been reading about all these upgrade swap issues. The first rep I spoke to was rude and dismissive; she said I would need to purchase two new SIM cards. I told her that per the AT&T upgrade guide on their website this did not appear to be the case - she did not appear to care. I ended up calling back to 611 and speaking with a much nicer rep. She initially told me the same thing as the first rep but when I asked about the info on the website (and read it to her) and about what the store said, she said that based on what the website said, I would be correct. She also put me on hold to check it out with her supervisors (whoa, follow up!) and she came back and told me that the info online was correct. I will be able to activate tomorrow at the Apple store, I will just need to call AT&T with the IMEI number first before Apple starts the activation process.
Although, I'm prepared to have issues tomorrow, at least I feel equipped with good information.