Index: _source/plugins/dialog/plugin.js
===================================================================
--- _source/plugins/dialog/plugin.js	(revision 4493)
+++ _source/plugins/dialog/plugin.js	Wed Nov 18 18:47:06 CST 2009
@@ -1712,8 +1712,10 @@
 							'top:0;' +
 							'width:100%;' +
 							'height: 100%;' +
-							'progid:DXImageTransform.Microsoft.Alpha(opacity=0)">' +
+							// IE6 opacity doesn't work on iframe element (#4517). 
+							( CKEDITOR.env.ie && CKEDITOR.env.ie6Compat ? 
+							'background-color:transparent' : 'progid:DXImageTransform.Microsoft.Alpha(opacity=0)">' ) +
-					'</iframe>' );
+							'</iframe>' );
 			}
 
 			html.push( '</div>' );