This page describes how to get Safari 4.0.4 running under MacOS X 10.5 to permanently accept self signed certificates. Other versions of Safari should be similar.
For an explanation of why you would want to do this (including when you should not do it) and instructions for other browsers, please see the main Self Signed Certificates page.
Steps for Safari:
Navigate to the web site in question.
You will see the warning Safari can't verify the identity of the website Click on the Show Certificate button:
You will now see a indicator that this is a self-signed certificate, with the expiry date:
Click on the Trust item to expand it:
Beside When using this certificate change Use System Defaults to Always Trust. The other two fields should update automatically:
Finally, Safari will ask you to provide your password before it will store the certificate. Enter your password.
On subsequent visits to this site, you should not see any warnings.