I notcied that this trick is good for the SSL stream between the client and IMAP server... however, does this also take care of the signing and (de)encrypting of email with the cert?

At work, we are requires to digitally sign our email, and cannot read others' email without their cert, etc. I have not been able to find a solution that does this on the Mac except for Netscape 4.x running in Classic mode (ick).

I would really love if Mail.app (or any other MacOSX mail client) could use certificates... THEORETICALLY Mozilla is doing this, but I would rather use Mail.app or whatever.

Anyone got suggestions?

You could use PGP...

http://www.sente.ch/software/GPGMail/index.html

works fine with Mail.app, since SSL support came in with 10.1.3, I'm feeling quite happy with Mail.app and this PGP extension to it.