auto-file-mode.el

(defun ps:auto-file-mode ()
  (when (save-excursion
          (save-restriction
            (widen)
            (goto-char (point-min))
            (save-match-data
              (looking-at "^#!"))))
    (let* ((cmode (file-modes buffer-file-name))
           (nmode (logior cmode (lsh (logand cmode #o444) -2))))
      (when (not (= cmode nmode))
        (message "Changing mode of script %s to %o"
                 buffer-file-name nmode)
        (set-file-modes buffer-file-name nmode)))))

  (add-hook 'after-save-hook 'ps:auto-file-mode)