You can find the full source code for this website in the Seam package in the directory /examples/wiki. It is licensed under the LGPL.
Yes, Seam provides excellent support for AJAX. First, Seam supports the ICEfaces and Ajax4JSF Ajax component libraries for JSF. If you prefer a more old fashioned
approach, Seam provides a complete JavaScript remoting framework which lets you call Seam components and subscribe to JMS topics directly from the client. Please refer to the Seam remoting
example application on how to use AJAX remoting to implement a chat room. Finally, Seam's concurrency model is designed especially for use with Ajax.