At the moment only four stores in the country are showing iPhones in stock, and only one of those stores has the black 16GB version...
This includes the number of iPhones available on the shelves at a particular store after 9 PM, but this does not take into account a delivery that may be received the next morning or afternoon which is sold before each store's available inventory on the shelves is updated or provided after 9 PM.
A store may show no inventory but receive a delivery of some stock the next morning. Between an Apple and at&t store, more than likely an Apple Store will receive additional inventory first, but direct fulfillment orders at an at&t store may take precedence over general inventory replenishment at either.
You can place a direct fulfillment order at an at&t store and then wait until the store contacts you when your order is shipped and when received at the store so you don't have to go to the trouble of showing up at an Apple Store in the hope that additional inventory was received that morning.