HTML Editor solutions usually rely on an iframe or div to be the actual html editor so they require a second hidden input field to be the input field that will post the value of the editor back to the server. This is normally not a problem because they usually update the input field before the form is posted. However the Microsoft AJAX UpdatePanel does not do normal form posts, so html editors like Fck and FTB don't get a chance to update their linked field. I have yet to see an official fix from the developers of both these solutions. Many people have asked "which editors work with UpdatePanel?", including the html editors that are not free including Telerik's RadEditor and Infragistics' NetAdvantage WebHtmlEditor.
Here is the C# code for the second solution (for the Page_Load event):
"for ( var i = 0; i < parent.frames.length; ++i ) if ( parent.frames[i].FCK ) parent.frames[i].FCK.UpdateLinkedField();"