If you're hoping to get your hands on a pair of iPhones on Friday, don't bother going to the AT&T store. You'll have much better luck at the Apple store.
Apple is also limiting sales of the iPhone, however, they will let you purchase up to two, rather than the 1 per person limit from AT&T. It's quite curious why the two stores would have different limits. I almost wonder if it doesn't have something to do with Apple's deal with AT&T. Perhaps they are hoping to get more customers to stop by the Apple store so that they will be exposed to more Apple products.
Unfortunately, your best bet to get an iPhone at all will probably be at an AT&T store, as I've been by several local stores that will be carrying the phone, and none of them have had anyone waiting just yet. So the question becomes, whether you should camp out at Apple for two phones and perhaps walk away with none, or swing by the AT&T store and have a decent shot at getting only one.