I use PHP sessions (not cookies, except for session id cookie) for all user data, and when a user goes to their profile user.mydomain.example they are immediately "logged out" until then remove the subdomain.
Is there a way to accept sessions from all domains as long as its *.mydomain.example
if(isset($_COOKIE['session_id']))
session_id($_COOKIE['session_id']);
Zend_Session::start(); //or session_start();
if(!isset($_COOKIE['session_id']))
setcookie('session_id', session_id(), 0, '/', '.yourdomain.example');
security be damned, if you are as frustrated with incomplete or bad answers as I am, this is your savior. It just works.