You can combine PlannerMode and the remind utility by using Please see remind.el for more details.

It would be interesting to combine Calendar, Planner and Remind in such a way that from CalendarMode someone could add appointments to .reminders

Another approach is to use to integrate your emacs diary entries with planner.

DanielMartins .emacs+remind configuration

;; ;; To include .reminders as a shell mode 
  (setq auto-mode-alist
        (cons '(".reminders$" . shell-script-mode) auto-mode-alist))

(add-hook 'sh-mode-hook
     (lambda ()
       (auto-fill-mode nil)))

;; From Sacha Chua

;; If you want your schedule to be automatically updated 
;; whenever you save a planner file, put this in your .emacs:

;; Remind options

(require 'remind)

(add-hook 'planner-mode-hook
   (lambda () (interactive)             
     (make-local-hook 'after-save-hook)
     (add-hook 'after-save-hook 'remind-parse-planner t t)
     (add-hook 'after-save-hook 'remind-export-to-diary t t)))

;; Alternatively, you can add the following lines to your
;; .emacs file:

(defadvice mark-diary-entries (before remind-generate-diary activate)
  "Generate a diary file from a .reminders file."