Index: modules/user/user.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.admin.inc,v
retrieving revision 1.33
diff -u -p -r1.33 user.admin.inc
--- modules/user/user.admin.inc	16 Nov 2008 15:10:49 -0000	1.33
+++ modules/user/user.admin.inc	12 Dec 2008 20:41:30 -0000
@@ -184,6 +184,8 @@ function user_admin_account() {
     $form['last_access'][$account->uid] =  array('#markup' => $account->access ? t('@time ago', array('@time' => format_interval(REQUEST_TIME - $account->access))) : t('never'));
     $form['operations'][$account->uid] = array('#markup' => l(t('edit'), "user/$account->uid/edit", array('query' => $destination)));
   }
+  // don't display a checkbox for user #1 as it is undeletable.
+  unset($accounts[1]);
   $form['accounts'] = array(
     '#type' => 'checkboxes',
     '#options' => $accounts
Index: modules/user/user.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.module,v
retrieving revision 1.944
diff -u -p -r1.944 user.module
--- modules/user/user.module	29 Nov 2008 09:33:51 -0000	1.944
+++ modules/user/user.module	12 Dec 2008 20:41:31 -0000
@@ -1572,10 +1572,17 @@ function _user_edit_submit($uid, &$edit)
 /**
  * Delete a user.
  *
+ * @note You can't remove user #1
  * @param $edit An array of submitted form values.
  * @param $uid The user ID of the user to delete.
  */
 function user_delete($edit, $uid) {
+  // User #1 can't be deleted
+  if ($uid == 1) {
+    drupal_set_message(t('Failed to delete user: This user is the superuser or root administrator, and cannot be deleted.'), 'error');
+    return;
+  }
+
   $account = user_load(array('uid' => $uid));
   drupal_session_destroy_uid($uid);
   _user_mail_notify('status_deleted', $account);
Index: modules/user/user.pages.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.pages.inc,v
retrieving revision 1.24
diff -u -p -r1.24 user.pages.inc
--- modules/user/user.pages.inc	24 Nov 2008 00:40:45 -0000	1.24
+++ modules/user/user.pages.inc	12 Dec 2008 20:41:31 -0000
@@ -238,7 +238,8 @@ function user_profile_form($form_state, 
   $form['_category'] = array('#type' => 'value', '#value' => $category);
   $form['_account'] = array('#type' => 'value', '#value' => $account);
   $form['submit'] = array('#type' => 'submit', '#value' => t('Save'), '#weight' => 30);
-  if (user_access('administer users')) {
+  // don't show delete button for user #1 as it is undeletable.
+  if (user_access('administer users') && $account->uid != 1) {
     $form['delete'] = array(
       '#type' => 'submit',
       '#value' => t('Delete'),
Index: modules/user/user.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.test,v
retrieving revision 1.22
diff -u -p -r1.22 user.test
--- modules/user/user.test	25 Nov 2008 13:14:29 -0000	1.22
+++ modules/user/user.test	12 Dec 2008 20:41:31 -0000
@@ -193,6 +193,41 @@ class UserDeleteTestCase extends DrupalW
   }
 }
 
+class UserRootAdminNoDeleteTestCase  extends DrupalWebTestCase {
+  function getInfo() {
+    return array(
+      'name' => t('Deny user #1 deletion'),
+      'description' => t('Assure that user #1 can\'t be deleted'),
+      'group' => t('User')
+    }
+  }
+
+  /**
+   * Tries to delete the root, which should be impossible
+   */
+  function TestRootAdminNoDelete() {
+    $rootadmin = user_load(1);
+    $this->drupalLogin($rootadmin);
+
+    // edit user page
+    $this->drupalGet('user/1/edit');
+    $this->assertNoRaw('value="' . t('Delete') . '"', t('Make sure the delete button doesn\'t get displayed on user/1/edit'));
+
+    // admin interface
+    $this->drupalGet('admin/user/user');
+    $this->assertNoRaw('name="accounts[1]"', t('Make sure you can\'t select user #1 for deletion on admin/user/user'));
+
+    // direct link
+    $this->drupalGet('user/1/delete');
+    $this->drupalPost(NULL, NULL, t('Delete'));
+    $this->assertText(t('Failed to delete user: This user is the superuser or root administrator, and cannot be deleted.'), t('Make sure it gives an error while trying to delete user #1'));
+
+    // even pure php shouln't allow it.
+    user_delete(1);
+    $this->assertTrue(user_load(1), t('User #1 is found in the database'));
+  }
+}
+
 class UserPictureTestCase extends DrupalWebTestCase {
   protected $user;
   protected $_directory_test;