Our way of turning TWiki into a task-tracking system actually consists of several custom plugins and tweaks, including

• Extending the forms functionality to create custom "task" types (comparable to Drupal's content types), allowing variables such as users and due dates to be associated with task pages.
• Email notifications of task creation and status/ownership changes.
• Periodic email notifications of overdue and soon-to-be-due tasks
• Greatly enhanced dynamic form handling — added metadata search functionality