How can I show/hide just one menu item based on user role?
You may try Menu per Role


  This module allows you to restrict access to menu items1 based on user
  roles. It depends on the Drupal core menu.module—just activate both
  modules and edit a menu item as usual. There will be a new fieldset
  that allows you to restrict access by role. The fieldset also appears
  in the node edit form.




D7 release is not stable yet its 7.x-1.x-dev