Index: wp-includes/default-constants.php
===================================================================
--- wp-includes/default-constants.php	(revision 13499)
+++ wp-includes/default-constants.php	(working copy)
@@ -288,7 +288,8 @@
 	 * Will be used as the fallback if the current theme doesn't exist.
 	 * @since 3.0.0
 	 */
-	define( 'WP_FALLBACK_THEME', 'twentyten' );
+	if ( !defined('WP_FALLBACK_THEME') )
+		define( 'WP_FALLBACK_THEME', 'twentyten' );
 }
 
 ?>