Ok, I wrote up a small hack, maybe this is already implemented but I
find it useful. Comments are welcome.

(defun mml-attach-pgpkey-mime ()
  "Short hack to attach the own public  `gpg' key."
  (interactive)
  (save-excursion
	(let* ((current (current-buffer))
		  (pgp-pubkey-temp-file (make-temp-file "pgp-public-key")))
	  (find-file pgp-pubkey-temp-file) 
	  (kill-region (point-min) (point-max)) ;just in case!
	  (pgg-insert-key)
	  (write-file pgp-pubkey-temp-file)
	  (switch-to-buffer current)
	  (mml-attach-file pgp-pubkey-temp-file "application/pgp-signature" "pgp-key" "attachment"))))