If you are seriously considering an arduino, then why don't you use one of the ethernet shields?
http://www.adafruit.com/index.php?main_page=product_info&cPath=17_21&produ cts_id=83
http://www.sparkfun.com/commerce/pro...oducts_id=9026
that would leave the serial port on the cRio open, and dwould allow a network switch to combine several on the robot.
The sparkfun shield communicates over SPI, which theoretically has a higher bandwidth then the serial comms that it is replacing