Index: user.module
===================================================================
--- user.module	(revision 173)
+++ user.module	(working copy)
@@ -653,6 +653,11 @@
     '#size' => 15,
     '#required' => TRUE,
   );
+  // Enable autocomplete
+  if (user_access('access user profiles')) {
+    $form['name']['#autocomplete_path'] = 'user/autocomplete';
+  }
+  
   $form['pass'] = array('#type' => 'password',
     '#title' => t('Password'),
     '#maxlength' => 60,