I would like to propose a new feature for Moodle 1.5 (and volunteer to implement it but only after Moodle 1.4 has shipped): the possibility to restrict resources or activities to a subset of students. This feature would be toggled on and off by a new icon that would appear behind the course module on the course home page in editing mode. Clicking this would lead to a page where the teacher can choose the students that should be allowed to view and use this coursemodule.
Internally this could be handled by an extra table course_module_students with fields 'id', 'userid', 'coursemoduleid'. This table would have to be consulted when deciding which coursemodules to show on the course home page for a particular user. Because this could slow things down a tiny bit there would be an administrative option to turn this whole feature off. This table would also have to be consulted when a module fetches its list of students (replacing get_course_students() by a new get_coursemodule_students()).