Yes, there's an iCalendar service that includes all tasks from all of your lists.
Calendar apps will normally only display events (and not tasks) on the calendar. For this reason, the Remember The Milk iCalendar service can export tasks as events so that they will show on your calendar. Only tasks with due dates will be shown as events.
The link for this service is available under 'iCalendar Events Service (All Lists)'.
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