From Daniel Larson’s Blog an article on the subject of Ajax in Sharepoint.
“In “Inside WSS” we did a chapter on using ASP.NET AJAX technologies within SharePoint. There are a few things we’ve learned since writing the book that we’d like to clear up, and a few ASP.NET techniques that we’d recommend avoiding in favor of more stable methods, as well as some additional tips and tricks for coding AJAX applications in the portal. ”
You can find this article here.