Are there any useful examples of node.js (such as a guestbook or chat room)?
Read about what I did to make it here: http://www.travisglines.com/web-coding/lets-make-a-twitter-clone-in-node-js
Check out the source code here: https://github.com/tglines/nodrr
There are some useful introductory videos on node at the Node.js Camp website that helped me get my head round things. Particularly the Introduction to Node video by Ryan Dahl (creator of Node.js) where he codes a basic chat server on the spot. Also the videos about Tim Caswell's game client are pretty impressive.
You may also want to look at the socket.io example on Tim Caswell's 'How To Node' blog if you're interested in a simple chat server.