Use attr_protected or we will hack you

Yes, but it gets protected automatically if you use the mentioned attr_protected or the accessible version.

Basic Rails code to update your own details on the 'view my details' screen is something like

    def update_details
      @logged_in_user = User.find_by_id(session[:userid])
      @logged_in_user.update_attrs(params[:user])
      @logged_in_user.save 
    end

Obviously you would never expose the :is_admin flag on the update_details screen, but a not even very cunning user could guess its there and manufacture up a post request containing the is_admin flag, setting it using the bulk assignment above.

Sticking in attr_protected :is_admin in your model means that ActiveRecord will just ignore that attribute if it is bulk assigned securing it with a single line of code.

Attr_accessible was new in Rails 2 (I think) - its says deny all BUT the ones listed - best way by far.