Good articles on SharePoint and AJAX:
SP1 should integrate AJAX so be careful until its released. Why? Read a blog from Fitz – snippet below:
We’ve been investigating this issue very carefully. We’ve been working directly with the ASP.NET team to come up with the nicest possible way of giving you the ability to make all of this work. But we’re not ready yet. We know some things work fine, other things will work fine once one or both of our teams have done some extra work, and a few (hopefully very few) things won’t be supportable in SharePoint sites.