I'm ordering an Ethernet Shield for a project I have planned, but before make the investment, I want to make sure the project will work. Eventually it will communicate with a website via ethernet, but I was wondering if I could test this through my PC, using it simply to route page requests, so that I can easily adapt it for the ethernet shield.
I'm not sure if the Arduino can do it, but you can look at the possibility of implementing SLIP. It's designed to layer TCP/IP over serial communications.
You can configure a PC as the SLIP server, for the device to login and route packets through.
Edit: Turns out there's an Arduino SLIP library that does that exactly!