Index: wp-admin/admin-ajax.php
===================================================================
--- wp-admin/admin-ajax.php	(revision 21992)
+++ wp-admin/admin-ajax.php	(working copy)
@@ -17,12 +17,19 @@
 define( 'WP_ADMIN', true );
 
 // Require an action parameter
-if ( empty( $_REQUEST['action'] ) )
+if ( empty( $_REQUEST['action'] ) && 'OPTIONS' != $_SERVER['REQUEST_METHOD'] )
 	die( '0' );
 
 /** Load WordPress Bootstrap */
 require_once( dirname( dirname( __FILE__ ) ) . '/wp-load.php' );
 
+/** Allow for cross-domain requests (from the frontend). */
+send_origin_headers();
+
+// Require an action parameter
+if ( empty( $_REQUEST['action'] ) )
+	die( '0' );
+
 /** Load WordPress Administration APIs */
 require_once( ABSPATH . 'wp-admin/includes/admin.php' );