That was fast... Jan Tielens has implemented the AjaxBasePart in the SmartPart web part to add AJAX functionality.
From Jan Tielens' Bloggings:
I've created a version of the SmartPart that makes use of the Eric's technique, let me introduce you the SmartPart with AJAX! So basically said: you can create a Web User Control (ASCX) with the Visual Studio Designer, that uses the ASP.NET AJAX extensions and run that user control as a SharePoint web part.
That's awsome! Be sure not to miss the screencast that Jan has put together, it does a pretty good job of showing some of the interesting things you can do with AJAX inside of a web part.
The SmartPart is an excellent tool, especially for casual developers that would like to develop SharePoint web parts but don't want to give up the designer experience that you get with normal ASP.NET development.
