The FCKEditor doesn't seem to be compatible with the RoundedPanelExtender of the AJAX Control Toolkit in MS Internet Explorer. Whenever I place the FCKEditor as a control within a panel that has the RoundedPanelExtender, the FCKEditor is visible for a second or two after page load and then disappears. It works perfectly well on Firefox though.
The browser versions that I'm using are MS Internet Explorer 7 and Firefox 3.