Adding CAs on OS X 2
My last entry was the way you do this pre-Panther (before OS X 10.3). Now, you can just double click the .pem or .crt file, and voila! The keychain opens, asking you which key to add it to. Select the X.509 Anchors keychain, and add away! So much simpler.
Also, since Apple Mail supports Keys and things you can now sign and encrypt mail using X.509 keys. Thawte and others offer these keys for free for individuals. This is a neat page showing how to get this working using Thawte as your CA.