I am writting becasue I have been working on a module that utilizes Ajax in Drupal via jQuery called "ActiveField".
The module is somewhat unique from other examples that I have seen in that:
1. It works with CCK content types while they are being created (i.e. before they are persisted to the database)
2. It works with existing themes and does not have it's own theme
The module is documented at: http://www.sageofcode.com/?p=59Read more