In my prior post I ran into a problem with FCKeditor on Apache having to do with serving JavaScript files as the proper MIME type. I ran into a similar problem with XML files when trying to load multiple instances of FCK textareas:
Error: this.DOMDocument has no properties
Source File: http://www.mysite.com/FCKeditor/editor/js/fckeditorcode_gecko.js
Line: 43
This is easily fixed in the same way, by telling Apache how to properly serve XML files. Put this in your httpd.conf or an .htaccess file:
AddType text/xml .xml