From http://www.mail-archive.com/gnu-emacs-sources@gnu.org/msg00034.html

randomize_region.el

;;; randomize-region.el --- randomize line order in a region

;; Copyright (C) 2005 Joe Corneli <[EMAIL PROTECTED]>
;; Copyright (C) 1986, 1987, 1993, 1994, 1995, 2003 Free Software Foundation, 
Inc.

;; Time-stamp: <jac -- Tue Apr 26 15:30:27 CDT 2005>

;; This file is not part of GNU Emacs, but it is distributed under
;; the same terms as GNU Emacs.

;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published
;; by the Free Software Foundation; either version 2, or (at your
;; option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING.  If not, write to the
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.

;;; Commentary:

;; Uses copied & modified versions of `shuffle-vector'
;; from cookie1.el and `reverse-region' from sort.el

;;; Code:

(defun randomize-region (beg end)
  (interactive "r")
  (if (> beg end)
      (let (mid) (setq mid end end beg beg mid)))
  (save-excursion
    ;; put beg at the start of a line and end and the end of one --
    ;; the largest possible region which fits this criteria
    (goto-char beg)
    (or (bolp) (forward-line 1))
    (setq beg (point))
    (goto-char end)
    ;; the test for bolp is for those times when end is on an empty
    ;; line; it is probably not the case that the line should be
    ;; included in the reversal; it isn't difficult to add it
    ;; afterward.
    (or (and (eolp) (not (bolp)))
        (progn (forward-line -1) (end-of-line)))
    (setq end (point-marker))
    (let ((strs (shuffle-list 
                 (split-string (buffer-substring-no-properties beg end)
                             "\n"))))
      (delete-region beg end)
      (dolist (str strs)
        (insert (concat str "\n"))))))

(defun shuffle-list (list)
  "Randomly permute the elements of LIST.
All permutations equally likely."
  (let ((i 0)
  j
  temp
  (len (length list)))
    (while (< i len)
      (setq j (+ i (random (- len i))))
      (setq temp (nth i list))
      (setcar (nthcdr i list) (nth j list))
      (setcar (nthcdr j list) temp)
      (setq i (1+ i))))
  list)

;;; randomize-region.el