Cheapest way to communicate with a Fez Panda II (or Arduino board)?
Have the serial out of one go to the serial in of the next, create a big loop.  Make your own simple packet structure and protocol.  If a board gets a packet that is not addressed to it it just passes it on.  Cheaper than usb or ethernet cable, no shields, very little software required.  if you do have long stretches then you might consider adding a differential pair, like an rs422 transceiver (422? 485? which is it?)