It's been a while, and to be honest, emacs-fu has been doing such a great job putting up useful snippets of Emacs information...
He just put up a question asking for folk's favorite dot-emacs tricks. I added my two cents with:
(defun tj-find-file-check-make-large-file-read-only-hook ()
"If a file is over a given size, make the buffer read only."
(when (> (buffer-size) (* 1024 1024))
(setq buffer-read-only t)
(message "Buffer is set to read-only because it is large. Undo also disabled.")))
(add-hook 'find-file-hooks 'tj-find-file-check-make-large-file-read-only-hook)