I'm attaching a new version of org-mime.el which incorporates Matt's
function below.  There are now two new functions, `org-mime-org-buffer'
and `org-mime-org-subtree' each of which takes a format argument
specifying the format of the final email, one of 'org, 'ascii, or 'html.

So, for example the following will export the current subtree as ascii
into an email body, using the MAIL_TO, MAIL_CC, and MAIL_BCC properties
to build the email headers, and the headline to set the subject (both
directly from Matt's function below).

(defun org-mime-subtree-to-ascii ()
  (interactive)
  (org-mime-org-subtree 'ascii))

Does this new version of org-mime look like it should be committed?  Are
there any features or changes that should be considered first?

Cheers -- Eric