With the new Safari in Tiger, it is easy to import self-signed SSL certificates you may come across. Just click Show More in the alert box, and then drag the certificate icon to a folder or your desktop. Then double-click the certificate, and Keychain Access will prompt to import it. Select the "X509Anchors" keychain from the Select box and click OK. Finally, enter your admin password to allow that keychain to be modified.
For example, my university has a self-signed SSL certificate for their IMAP server, and Mail.app constantly complains about this. So I pointed Safari at the IMAP SSL port eg: https://imap.ufl.edu:993/ to grab and import the certificate into the system keychain. Now when I start up Mail.app, it doesn't complain when connecting securely to my school's IMAP server.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050503172154759