Index: wp-includes/js/tinymce/tiny_mce_config.php
===================================================================
--- wp-includes/js/tinymce/tiny_mce_config.php	(revision 7585)
+++ wp-includes/js/tinymce/tiny_mce_config.php	(working copy)
@@ -49,11 +49,11 @@
 $https = ( isset($_SERVER['HTTPS']) && 'on' == strtolower($_SERVER['HTTPS']) ) ? true : false;
 	
 $baseurl = get_option('siteurl') . '/wp-includes/js/tinymce';
-if ( $https ) str_replace('http://', 'https://', $baseurl);
+if ( $https ) $baseurl = str_replace('http://', 'https://', $baseurl);
 
 $mce_css = $baseurl . '/wordpress.css';
 $mce_css = apply_filters('mce_css', $mce_css);
-if ( $https ) str_replace('http://', 'https://', $mce_css);
+if ( $https ) $mce_css = str_replace('http://', 'https://', $mce_css);
 
 $mce_locale = ( '' == get_locale() ) ? 'en' : strtolower( substr(get_locale(), 0, 2) ); // only ISO 639-1