What a nuisance.
Apple has a widget that works only after 9pm (local time?) to check local Apple Store stocks:
Apple Retail Store - iPhone availability at the Apple StoreI'd prefer to buy at an AT&T store due to the tactical complexity of AT&T's deliberately evil contracts, but I suspect Apple stores are getting more shipments. AT&T is not offering a similar availability widget, they suggest payment up front and they'll hold a phone when it arrives. Fortune reports they have no phones anywhere, and no word on when they'll receive any.
Check availability after 9:00 p.m. the night before you plan to visit an Apple Retail Store.
Get there early. Shipments arrive most days, but be sure to arrive early since iPhone 3G is sold on a first come, first served basis.
I'd pay to reserve at an AT&T except I know AT&T is so Satanic that Hell itself could not abide them. (Apple, on the other hand, is merely a close confidante of Beelzebub. AT&T store staff, in my experience, are quite good btw.)
Update 7/16/0810pm: none at all