What you need is a command to bind the incoming telnet session on your unix system to the (usb) serial port that has the arduino on it. A sort of one-port "terminal server." It looks like "nc" (netcat) might be part of standard linux distributions, and might do what you need. See also "pastel", and presumably others.
I am sorry that I did not clarify. The linux system running the control scripts will be the same system with the arduino connected to it. So technically there will be no network data needing to be passed to the arduino. The only exception being if/when I put an xport on the arduino of course.
The difference would be that on the local USB connection I would be telneting to the local serial port. With the xport I would be telneting to the IP interface of the xport.