Can the NFC hardware within Lumia 920 emulate a 125 kHz Proximity Access Card?
It looks like the NFC hardware implements the standard that is a superset of the standard that access cards use. But I don't have enough knowledge of those radio standards to understand if a phone can work only as a receiver or also as a transmitter of such signals.
I will also appreciate a link to a good overview article that explains those standards in simpler terms than the official specifications.
Currently, card emulation is not supported by the Windows Phone proximity API.
Sources: