Ok, at least it sign/encrypt mail now.  I've placed smime.el and
mml-smime.el in the contrib directory in cvs. Thought I should commit
it, so that someone can contribue password handling stuff for openssl
while I'm on holiday. :)

Installation (but read smime.el):

(require 'mml-smime-setup)
(mml-smime-setup)
M-x customize RET smime RET

Use:

Add "postprocess=smime-sign" tags to parts, preview with C-u M-m P.