This http://www.westermo.net/Resource.phx/content/products/fieldbus/converters/fd-40.htx on the other hand will probably do what you need.
He wants to connect to a profinet (physical medium: Ethernet) not a profibus, that's why he wanted to use an Arduino Ethernet, that one has the Ethernet interface onboard but no USB connection to the host, so the idea is strange anyway.