What happens if you leave out an `access callback` specification? In a custom module, if you have a menu hook item that has a custom access argument, but don't specify an access callback, what is drupal's default behavior? Does it use user_access? In other words, is an access callback necessary to specify?
Yes it uses user_access() by default, but only when you specify 'access arguments'. See http://api.drupal.org/api/drupal/modules--system--system.api.php/function/hook_menu/7.