This binds capital oh, or 'O', in dired to open as if double clicking
in the finder the file under the point.

;;; Dired enhancment.

(defun macosx-click2-file (filename)
   (call-process "open" nil 0 nil filename))

(defun dired-macosx-click2-file ()
   "In dired, open this file as if the user doubled clicked it in the
finder."
   (interactive "")
   (macosx-click2-file (dired-get-filename))
   (message "Opening..."))

(add-hook 'dired-load-hook
          '(lambda ()
             (define-key dired-mode-map "O" 'dired-macosx-click2-file)))