The compatibility issue discussed in this thread between the Ajax Control Toolkit and IE9 is now fixed with changeset d2c643a4ff62

You can download the fix here:

http://ajaxcontroltoolkit.codeplex.com/SourceControl/changeset/changes/d2c643a4ff62

http://stephenwalther.com/blog/archive/2011/04/18/new-april-2011-release-of-the-ajax-control-toolkit.aspx

http://stephenwalther.com/blog/archive/2011/05/09/may-2011-release-of-the-ajax-control-toolkit.aspx