Just as the rumors suggested, Apple updated the official Apple Store app to include support for customer check-ins, reservations and other location-based services.
You can use the app to reserve products to pickup at Apple retail stores, check into stores upon arrival to alert employees to your appointment and add reservation appointments to iCal (note that iOS 4 is required for check-in and iCal support). You'll also be able to add engraving and gift wrapping options when making purchases from the store.
