Randy Sarafan, technology editor at Instructables, made a Arduino Ethernet Shield Tutorial. There are two examples show how to use it to serve HTML pages, and parse URL strings.
The Arduino Ethernet Shield allows you to easily connect your Arduino to the internet. This shield enables your Arduino to send and receive data from anywhere in the world with an internet connection. You can use it to do fun stuff like control robots remotely from a website, or ring a bell every time you get a new twitter message. This shield opens up endless amounts of possibility by allowing you to connect your project to the internet in no-time flat.