On the 'replace other headers', the problem there is that we end up back in the rat-hole. People will propose all sorts of random headers ad infinitum.
And others will counter that there are integrity problems and then we have the interop issue, etc.
I don't think that the problem is big enough to require a whole new S/MIME spec to solve, just a minor tweak to implementations.
--Paul Hoffman, Director --Internet Mail Consortium