What do the 2 pins on the back of the Nexus S do?
This is for the NFC functionality. Those contacts are used to transmit the info to and from the receiver/transmitter on the back cover (seen below outlined in green, though you probably guessed it :) ). They have to put it on the back cover because NFC only works for usually no more than a few centimetres, and they need a way to feed the information back to the phone system itself.

It is discussed more in this post on another Android forum.