updated 12:51 pm EDT, Mon September 23, 2013
Ties in with new Personal Pickup availability
The official Apple Store app has quietly gained the ability to show iPhone stock at local outlets. The option is buried within the checkout process; shoppers have to tap a "Check Avaibility" button after putting an iPhone into their cart. The addition appears to have been made in tandem with the opening of Personal Pickup for the iPhone 5s and 5c.
Stock levels are inconsistent. Although a number of US stores are largely sold out of new iPhones, some shops do have limited numbers of a few models ready for same-day pickup. The situation should improve later this week as resupplies arrive from China.