UNITED AIRLINES is planning to install steel cable barriers on its aircraft to block access from the passenger cabin to the flight deck when the cockpit door is open. This is the first time that a major US carrier has used its own initiative (rather than being forced by government) to restrict cockpit access in the post September 11 period. The airline has been using catering carts to block the cabin access when the doors have been open. The Federal Aviation Administration has approved the modification which might very well be copied and could become US transportation policy. El Al use a more sophisticated door system to stop interlopers.
http://www.ual.com