Safari doesn't trust the certificate, Camino does. The site has many users.
This isn't a WebKit issue. Certificate validation is handled by the security libraries that ship as part of Mac OS X. The error indicates that the certificate is signed by a certificate authority that does not have their root certificate installed on Mac OS X. You should file a bug report at <http://bugreport.apple.com/> so that the appropriate people at Apple can look in to this.
Submitted at<http://bugreport.apple.com/> as Bug ID# 6953402