A big thank you to Sam Marshall of The Open University for developing a useful new feature in Moodle 2.0 - Conditional activities. This feature enables teachers to restrict the availability of any activity according to certain conditions, such as dates, grade obtained, or activity completion.
For further details, including links to two demonstration videos, see the Conditional activities documentation.
As always, help in testing new features (as well as bug fixes and improvements) is much appreciated. Please report any Conditional activities issues in the Moodle Tracker.