As of Janurary 2010, there is no UI for installing certificates. It can be done using command line tools. The following example assumes you have a Certificate Authority that needs to be added, as well as a personal certificate.
$ apt-get install libnss3-tools
$ certutil -d sql:$HOME/.pki/nssdb -A -t "C,," -n "Company CA" -i ca.company.com.crt $ pk12util -d sql:$HOME/.pki/nssdb -i user-certificate.pfx
Finally, until there is a UI for selecting which certificate to use the following argument must be added to the launcher.
--auto-ssl-client-auth
Add the same argument to the browser command in gnome-default-applications-properties.