Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 21203)
+++ wp-includes/ms-functions.php	(working copy)
@@ -1084,7 +1084,7 @@
  */
 function domain_exists($domain, $path, $site_id = 1) {
 	global $wpdb;
-	return $wpdb->get_var( $wpdb->prepare("SELECT blog_id FROM $wpdb->blogs WHERE domain = %s AND path = %s AND site_id = %d", $domain, $path, $site_id) );
+	return apply_filters( 'domain_exists', $wpdb->get_var( $wpdb->prepare("SELECT blog_id FROM $wpdb->blogs WHERE domain = %s AND path = %s AND site_id = %d", $domain, $path, $site_id) ), $domain, $path, $site_id );
 }
 
 /**