--- sites/all/modules/domain/domain.module copy	2008-08-26 14:20:02.000000000 -0700
+++ sites/all/modules/domain/domain.module	2008-08-26 14:20:12.000000000 -0700
@@ -469,7 +469,7 @@ function domain_default($reset = FALSE) 
   if (empty($default) || $reset) {
     $default['domain_id'] = 0;
     $default['sitename'] = variable_get('domain_sitename', variable_get('sitename', 'Drupal'));
-    $default['subdomain'] = variable_get('domain_root', '');
+    $default['subdomain'] = variable_get('domain_root', strtolower(rtrim($_SERVER['HTTP_HOST'])));
     $default['scheme'] = variable_get('domain_scheme', 'http');
     // Set the valid flag.
     $default['valid'] = TRUE;