auth managers expose isTokenValid method - if you are using grails REST spring security will call the
 validate token endpoint (requires, 1.4.0.RC5+ of the grails rest spring security plugin). If the token is invlid
 it removes it from the browser (this allows turning on anonymous access - if you send an invalid token
 the backend will send a 403 regardless if the resources is setp to allow access to all)