We have a really nice forum/CMS/Blog/eCommerce/image library, application that uses Ajax extensively and uses the 3x version of FreeTextBox in Ajax mode. We will make version 4.0 work with Ajax too. Check it out at http://bizmall.ph. Our support forums demonstrate this directly.
Our system also uses the Nitobi Ajax components such as Grid, drop down combo, etc to make for a nice user experience when managing content, products, images, etc.
We even have a nice CAPTCHA that works well in Ajax and can dynamically show or not show depending on the situation like an authenticated user versus an anonymous user.
If you need help building your ASP.NET 2.0 Ajax application, let us know by contacting us through bizmall.ph.
Overall, freetextbox is a great product and works nicely in our application too!