Oopsy! Someone at A&T didn't get the memo. Yes, the iPhone 4S is all new and fast, but it is not 4G compatible, unlike what AT&T currently advertises on its website.
If you go to this page on AT&T's site, you will get a list of 4G devices. The first one at the top of the list is the iPhone 4S, which we know only supports 3G at this point...