It's not entirely clear from your question whether you want an iPhone app to detect the devices, or the devices to detect the phone.
If you have the former in mind, your requirement is exactly what BtLE's (Bluetooth 4.0/Smart/LE) advertising capability is for. Apple's iBeacon technology is such an implementation, or you might roll your own. It'll allow you to do fine grained location in an iOS6+ app running on 4S, 5, and recent iPads. BtLE advertisement detecting apps will easily pass App store guidelines.
From the other direction, devices with BT 2.x - 4.x radios, or with wifi radios, or with various scanners and software radios, will be able to detect mobile phones, tablets, and other radio equipped devices. How much can be discovered is wildly variable.