Wow, has everyone had nothing but bad experiences with AT&T? I had quite the opposite experiences, AT&T = good and Apple Store = bad.
AT&T - I showed up at the store and said, "I want to upgrade my iPhone to the new iPhone 3G". This was shortly after it was released, sometime in late July. The rep gave me a sheet with all the plan options to choose from while he went to the back to get the model I wanted. When he came back it put all my selections into the computer. He opened the iPhone 3G and activated it. After a total of 7 minutes I walked out the door and my new iPhone 3G was fully functional.
Apple Store - (I went with 2 friend's who were also going to upgrade their original iPhones to the iPhone 3G, this was in mid-August). We arrive at the Apple Store at King of Prussia Mall in Philly, it is absolutely packed. There is a line of 3 people outside waiting for iPhones. We get in line, after 20 minutes of seeing no one, someone comes out and takes in the next guy in line and says he or someone else will be right with us. After another 15 minutes someone comes out and does "pre-approval" and gives my friends these cards stating which model they want and they they are pre-approved. 30 minutes later it is finally their turn (after 3 people were ahead of us). We go into the store and they sit them down by a seemingly random computer on one of the tables. Comes back with the iPhones 10 minutes later. He starts doing the upgrades on the handheld registers they have, entering their account info and pulling up the pre-registration, and going through the different plan options. After another 20 minutes both phones are activated and working. One more signature and they are done. We leave and it has taken forever. Just to check on our way out we stopped in an AT&T store, they said they only had 8GB models anyway.
I don't know but after that experience at the Apple Store I do not see how anyone could prefer that. Every Apple Store I have been too is like that, there are 3 near me that are all an hour away at least.
Also not sure what it would be like at Best Buy, I don't know anyone who has gotten it there. One advantage might be that they might sell you a replacement plan (from Best Buy) on it so you can have some insurance if you break it.