That's great, thank you. I have this, but it doesn't work:

(defun filter-scheduled-todo-tasks (content backend info)

  "Filter iCalendar export to include only TODO tasks that are

not done, but which are scheduled or have a deadline."

  (when (eq backend 'icalendar)

    (if (and (org-entry-is-todo-p)

             (not (org-entry-is-done-p))

             (or (org-get-scheduled-time (point))

                 (org-get-deadline-time (point))))

        content nil)))

... called with:

(let ((org-export-filter-final-output-functions

         '(filter-scheduled-todo-tasks)))

    (org-icalendar-combine-agenda-files))

I have (setq org-icalendar-include-todo t) too.

Using edebug, it seems that the `content' argument only iterates through the top-level headings of each of my agenda files. I was assuming it'd iterate through each subheading too --- do I need to do this manually?