Credit Cards have "blink" RFID chips, which aren't too far off from NFC tech. It's tiny, taking little space which is why it's been widely used for many years in Japan and the E.U. in many mobile devices.
Sources? Sorry, not "Deep Throat"
I can tell you that [as with any iPhone release] many prototypes were tested, and NFC was implemented in just about every prototype. If you've been following the discussion simply online, you'll have learned that Master Card was very vocal about NFC deployment with the release of a popular mobile phone. A simple google search will lead you to the quotes from various executives regarding the matter just before the iPhone 5 was released.
Someone joked that Apple demanded 30% on all iPhone NFC transactions with Visa and MasterCard, you aren't that far off.
For those stating NFC is more difficult than a RFID, read on the matter. Here's a very simple start:
Difference Between RFID and NFC
The main reason [and I'll refrain from stating "only reason" for those who are keen to attack on MacRumors] that Apple released a non-NFC iPhone was due to their demands with the credit companies. MasterCard has been on record that they had NFC systems set for deployment, such a brazen multi-million dollar investment surely wouldn't hinge on mere speculation.