Login Banner for users
"This system is for authorized users only. Individual use of this computer system and/or network without authority, or in excess of your authority, is strictly prohibited. Monitoring of transmissions or transactional information may be conducted to ensure the proper functioning and security of electronic communication resources. Anyone using this system expressly consents to such monitoring and is advised that if such monitoring reveals possible criminal activity or policy violation, system personnel may provide the evidence of such monitoring to law enforcement or to other senior officials for disciplinary action."
However, nobody reads it and, of course, the longer it is the less chance there is of anyone reading anything like this. These sort of disclaimers serve no real legal purpose (kinda like the legal disclaimers regarding 'destination recipient only!' at the end of some emails) but at least give fair warning to anyone casually trying to access a system without permission.