Because a Big Brother Database of my contacts isn't complete if I
don't keep track of what e-mail I sent them and when I sent it, this
bit of Emacs Lisp code adds Gnus subjects to the BBDB records of the
people to whom I sent e-mail.

(defun sacha/gnus-add-subject-to-bbdb-record ()
  "Add datestamped subject note for each person this message has been sent to."
  (let* ((subject (concat (planner-today)
                          ": E-mail: " (message-fetch-field "Subject") "\n"))
         (bbdb-get-addresses-headers
          (list (assoc 'recipients bbdb-get-addresses-headers)))
         records)
    (setq records
          (bbdb-update-records
           (bbdb-get-addresses nil gnus-ignored-from-addresses 'gnus-fetch-field)
           nil nil))
    (mapc (lambda (rec)
            (bbdb-record-set-notes rec
                                   (concat subject
                                           (bbdb-record-notes rec))))
            records)))
(add-hook 'message-send-hook 'sacha/gnus-add-subject-to-bbdb-record)

It should be really easy to set up Gnus to expand some kind of
!followup macro into a TODO item in my planner and an “I hope to hear
from you by ….”. Ridiculously easy with Emacs Lisp and an insanely
customizable editor, but I might not have enough battery life. I've
got 28 minutes, and then I'm off PC for a while.

/mnt/media/sacha/notebook/emacs/dotgnus.el