Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 12249)
+++ wp-includes/functions.php	(working copy)
@@ -2718,6 +2718,7 @@
  * to be merged into another array.
  *
  * @since 2.2.0
+ * @uses apply_filters() Calls 'wp_parse_args' hook with return value as well as $args and $defaults.
  *
  * @param string|array $args Value to merge with $defaults
  * @param array $defaults Array that serves as the defaults.
@@ -2732,8 +2733,9 @@
 		wp_parse_str( $args, $r );
 
 	if ( is_array( $defaults ) )
-		return array_merge( $defaults, $r );
-	return $r;
+		$r = array_merge( $defaults, $r );
+
+	return apply_filters( 'wp_parse_args', $r, $args, $defaults );
 }
 
 /**