What's new in trunk: Subtasking
To start adding subtasks, you need to enable the new 'Manage subtasks' permission. Then, go to an existing issue and hit 'Add' in the subtasks section. Priority, start and due dates, progress, estimate, spent time are automatically rolled-up to parent issues.
The gantt chart was adjusted to display the issue hierachy:
A 'Parent task' column can be added to the issue list. Sorting by this column will make the issue list display the issue tree as well.
This new feature will be part of Redmine 1.0.0 (RC) release.