Rob Gray from RobGray.com has been developing a substaintial RS485 networking hardware for Arduino.

http://busnet.robgray.com/hardware/arduino-dongle/index.php

From the site:

“

BUSnet is a low‐speed control network designed to monitor and control devices incommercial or light‐industrial environment but equally suitable for otherapplications. It is a multi‐master point‐to‐point protocol using a ring topologywhere all nodes can transmit frames when they please.

“