signed SMIME verification

I've been working a bit on SMIME with M2Crypto recently, and I noticed
there's no recipe in the M2Crypto docs on how to verify a signed mail
without knowing the signer's certificate.

Openssl's "smime verify" command is able to do that, and the same thing
can be accomplished with M2Crypto. I've wrapped up my findings in a
simple class which can be found at

http://www.asiatica.org/~ludo/archive/2004/05/Python_smime_verify.html

Please let me know if there are errors/inconsistencies or better ways of
doing this.

Ludo