On a side note, if anyone is looking to build something like this, please consider building a low-end device (running on Linux) with 16 or 32 RS-232 ports that I can SSH to...
Other than that, give it two USB ports so that the end user can connect a wireless USB NIC and/or a 3G/cellular card if they need to. Two (at most) 10/100 or 10/100/1000 Ethernet ports would be perfect. Video (VGA/HDMI/whatever) wouldn't be necessary, nor would sound.
Keep it under $200 and I imagine the networking industry will be happy to take them off your hands real quick.