How would one facilitate communication between devices based upon proximity given a particular app? Is it possible at this point? For example, if I have an iPhone or an iPhone and an Android based device, can I allow these devices to somehow transmit some beacon to each other so that they are able to recognize each other via an application or to at least know that they need to speak to a server at this point and transmit coordinates?
If the proximity requirement is of the order of few centi-meters then NFC (as mentioned by garzanti) is a good choice, (NFC is still a new technology being adopted in Mobile phones so current install base is limited) If the proximity requirement is of the order of few feet , then the best choice is Bluetooth, Bluetooth is available in a large number of devices/phones. Bluetooth has the initial inquiry/discovery process which is a beaconing process to discover nearby devices in vicinity to connect to, with this is is possible to implement and detect proximity of devices. On android this is fully supported and can be implemented easily. With iPhone Bluetooth public APIs are limited to only Gamekit , Gamekit uses internal automatic mechanism to detect devices in proximity running the same applications. Depending on your usage requirements it may be possible to do what you are thinking using the iPhone Bluetooth GameKit Apis, iPhone also has MFi program allowing use of Blueototh to develop accessories using Bluetooth and will have more detailed access into the iPhone Blueooth capabilities/apis , to use this you need to part of the Apple MFi program to get access. See here for MFi details: enter link description here