If you enable iChat encryption, .Mac will generate a Certificate and key with your .Mac name and store it in your keychain. This is used to encrypt iChat text and video, and it works well.
But did you know ... you can also use your .Mac certificate to sign and encrypt email? To do this, open Keychain Access (in Applications/Utilities), and check that your .Mac certificate is there. Then open the Preferences panel in Keychain Access and check the Search .Mac for Certificates box.
You can then start Apple's Mail app, and choose your .Mac email account as the sending account in a new message. When you do, you will see icons (as seen to the right) above the message window to toggle on/off encrypting (the lock icon) and digital signing (the checkmark) of email. Remember, though, to encrypt email to another person, you must have their certificate in your keychain.
[robg adds: I tested this with another .Mac user, and it worked as described.]

