Dreamhost has finally implemented the one click install of Trac a while ago (so we no longer have to go through a very long process to install a trac). It is done through FastCGI, and it start with no Authentication configured.
To setup authentication, simply use the .htaccess Auth in /trac install directory (the full path would be given by the email Dreamhost sends out on successful setup of Trac, when it tells you to change the trac.ini file)