Nice hint at MacOSXHints about using the certificate that encrypts iChat sessions in Tiger to authenticate and encrypt mail as well. Timely for me; I haven’t gotten around to renewing my existing certificate that I set up with Thawte, and I think it expires soon (if it hasn’t already).
See my earlier post about issues with signed mail and a link to the process for getting a personal certificate for digital signature use from Thawte.