Modifying mobile device operation using proximity relationships

Described is a technology by which a mobile computing device such as a mobile telephone operates differently based on detected proximity to another mobile device with which the first mobile device has a defined relationship. For example, the first mobile device may transfer content to the second mobile computing device when proximity corresponds to a non-cellular communications coupling, such as Bluetooth® or Wi-Fi coupling. In this manner, content transfer is deferred until a non-cellular coupling exists. The mobile device (or both devices) may output a notification to indicate that the other mobile computing device has been detected within a threshold proximity, such as via GPS data or by being within the same cellular tower. The type and/or settings of an output notification may vary based on different thresholds.