I was browsing the CodePlex website (dasBlog is considering switch from SourceForge) and saw there was a new release of the Ajax Control Toolkit.  It looks like the new build (10606) has quite a few bug fixes.  If you haven't checked out the controls its well worth the time.  Even if the controls don't do exactly what you are looking for they provide a great example of how extenders work and the AJAX Extensions in general.