Well, I finally got around to getting a free certificate and enabling Mail.app to be able to sign and encrypt email.
[This article][1] was invaluable.
**Key point:** If you get a certificate from [Thwate][] you must use [Firefox][], Safari doesn’t work.
[1]: http://www.macdevcenter.com/pub/a/mac/2003/01/20/mail.html
[thwate]: http://www.thwate.com
[firefox]: http://www.firefox.com