So I have arduino UNO and the Ethernet sheild. I've been able to get a webpage to load with an image and a link to another page. What I'm having trouble with is finding code that will provide a guest book form and then display entries on either the same page or a separate page. Is there a way to enable "world write permission" for an HTML guest book, or is there some other form of coding that the UNO can handle that will do what I'm looking for?
OK you want arduino to record guest submitted comments? You can record that on an SD card in a file. Then every time you web page is requested, send the entire content minus the end of the document, open the SD file, send it through (with some additional markups), and then send the end of your original web page html.
You should search for how to handle http GET or http POST with arduino web server. These are the two ways that a web browser uses to send data to arduino when you press a submit on a guest comment form. There are many examples but you will have to adept.
Most of the functionality is the same with Arduino as it would be with any other web server - you need to implement a form which enables users to submit new comments. The Arduino doesn't come with a web app framework so you will need to deal with the form submission yourself and extract the comment text from the request. You should have no trouble finding other examples of extracting parameters from requests, and you may even find examples that show how to handle form submissions. If not, just print out the whole request and it should be obvious where your comment text appears in the request. Then you need to write some code to extract it from the request.
Having got the comment, you will need to save it. Feasible places to save it would depend on what volume of comments you expect to support, but saving them on an SD card would probably scale for anything your Arduino would be suitable to host.
Finally you will have to write the code to insert the existing comments into the page in the appropriate place - that's a web design issue.