I actually found this completely by accident, I was trying open a web page in OmniWeb (shift-command-U), when I missed and pressed option-command-U instead with a message open.
The result is that you see the complete email message completely unrendered. The graphic file attachments are still base64'd, and none of the HTML links within the message are active.
Also, all of the control characters are highlighted... for those of you who really want to see all the details of your email messages within Mail.app
[Editor's note: Just when you think you've seen all the Mail tips possible ... as the comment points out, it's in the menus, but deeply buried. I had no idea it was there!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020717021512399