The Battery Door only holds the antenna, the NFC radio is on the board like all the other chips. most non built in NFC devices are not going to be attached to the phone, and some refer to these type of devices as a "Puck" this format was popularized between 2005-2007 when many mobile phones did not contain true GPS, and many smartphones could connect to and utilize GPS receiver units through bluetooth.
So which is it then folks? Is it a chip that is built into the phone, with antennas in the battery cover like the NS4G or is it all (chip and antenna) contained in the battery cover?
The Nexus S 4G has the chip ON THE MAINBOARD of the phone, with two dipoles on the back of the device. These connect to the antenna in the battery cover on the back of the phone. I wonder if the Galaxy Nexus is any different?