Ernie the Attorney talks about encrypting E-mail.

I tried to use the PGP encryption technology with a tech-savvy friend once. I won't describe the whole process but if you are interested click here. The important thing to understand is that, even if you can figure out how actually encrypt and decrypt emails, part of the process involves creating a special private/public key. In short, it's a pain in the ass to encrypt emails, even if you know what you are doing. After having done it myself I have come to the conclusion that the only people who are routinely encrypting emails are either uber-geeks or criminals/terrorists. [Ernie the Attorney]

Setting up mail encryption on OS X using Mail.app is actually not too difficult. MacDevCenter has a pretty good article about it (and you can now use Safari instead of Mozilla to request your certificate (make sure to read through the comments at the end)). Once you have it installed it is just a click of a button to sign your messages with your certificate, and if you have received email from someone else with a cert you can encrypt it with the click of another button.