Yet another Mail.app attachment-dragging hint:
You can create a new message with an attachment from another message just by dragging the attachment from the one message to the Mail.app icon in your dock. A new message opens up with the attachment pre-attached.
This of course is hardly different from dragging a file from the Finder to the Mail icon in the dock. But it's a handy one-step maneuver if, for example, you need to forward artwork from someone on your staff to one of your clients without your artist's commentary.
I only mention it because it seems a little strange to drag something from an app back into the same app, but hey, it works.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060308190103420