I am currently working with the NFC Shield and have successfully implemented full peer-to-peer communication according to NFC-forum standard. At least it can send and receive NDEF messages to a Samsung Galaxy Nexus. I will report hints on where you can find information about LLCP, SNEP, NPP and NDEF wich are all a part of the protocol standard.
I have a question about the actual NFC Shield. Is there any reason why you use TXB0104PWR for voltage leveling? Doesn’t PN532 have a separate powersource (DVDD) for the digital voltage level?