Firstly, we should note that RFID is a catch-all term that describes a vast array of technologies and standards. RFID tags can be relatively large and battery-powered, such as ones used in toll collection, to small "passive" tags that can be embedded into credit cards, drivers licenses (called "Enhanced Drivers Licenses" in the U.S.), passports, or stuck onto a piece of merchandise.
Currently, cell-phone usage of RFID technology is centered around Near Field Communication (NFC). NFC has three main usage scenarios: a phone acting as an RFID tag; a phone acting as an RFID reader; and peer to peer communication (P2P).
In RFID tag mode, a phone could be used as a payment device (like a credit card), an identity card, or act as a car key. In RFID reader mode the phone would be able to interact with tags in its vicinity. This article and video (below) demonstrates how an iPhone with RFID could use physical objects to control media playback. And in P2P mode, Bluetooth pairing can be streamlined.
These are just a few ways that RFID could be used in an iPhone. When or if it becomes a reality isn't clear, but hopefully now you have a better idea of what the potential is for Apple's research in this area.