Before creating cas auth accounts in moodle directly, show some information to users and ask for their confirmation.

ask_before_creating_cas_users.patch

Index: moodlelib.php
===================================================================
RCS file: /cvsroot/moodle/moodle/lib/moodlelib.php,v
retrieving revision 1.960.2.168
diff -p -r1.960.2.168 moodlelib.php
*** moodlelib.php       23 Feb 2011 20:23:52 -0000      1.960.2.168
--- moodlelib.php       8 Apr 2011 00:05:59 -0000
*************** function authenticate_user_login($userna
*** 3226,3231 ****
--- 3242,3255 ----
                  $user = update_user_record($username, get_auth_plugin($user->auth));
              }
          } else {
+             $cas_create_new = optional_param('createnewcas', 0, PARAM_BOOL);
+             if (!$cas_create_new) {
+                 echo '<p>这是您第一次通过CAS认证访问乐学网。</p>';
+                 echo '<p>如果您在乐学网已经有账号，务必先<a href="/bind_cas_users">绑定CAS和本站旧账号</a>，否则已有个人数据（选课、成绩、作业等）都会丢失。此操作只需一次。</p>';
+                 echo '<p>如果您是乐学网的新用户，可以直接建立新账号。</p>';
+                 echo '<p><a href="/bind_cas_users">我是老用户，要做绑定</a>&nbsp;&nbsp;&nbsp;&nbsp; <a href="?createnewcas=1">我是新用户，建新账号</a></p>';
+                 die;
+             }
              // if user not found, create him
              $user = create_user_record($username, $password, $auth);
          }