In Mail.app, drag a received message from your In box (or any other box, for that matter) to the Drafts box.
Now, when you open the message, it will be fully editable. You can then send it off, just as if you had made a new email. No copy and paste required!
[robg adds: If you want a copy of the message to work on, instead of the original, just Option-drag it to Drafts.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050224164307315