I have run into an issue with the Explore NFC board where the I2C pins are bridged to the "ALE" pin on the PN512 and another GPIO pin on the RasPi, even when in it's SPI configuration. It seems NXP don't want you also using the I2C bus on the Pi while you use this dev board (there are zero-ohm links to swap connectivity and addressing). 

I will have a basic schematic and fix for the dev board shortly since NXP don't supply such info. I will also have a draft shield hardware design this weekend. 

Since I am using SPI for the NFC board (I want to leave this separate to other peripherals to keep it fast and disruption free!) I will be using I2C for the rest of the peripherals. This is because they will only be operating one at a time in sequence for small transactions, so they can share a slower bus without issues.