http://www.cups.org/documentation.php/spec-ipp.html states that cups also supports 'https' for printing via IPP, but the symlink from /usr/libexec/cups/backend/https -> /usr/libexec/cups/backend/ipp is missing. Creating the symlink manually makes it possible to print over https.
Added a patch to add another symlink for https in cups-1.3.7-r1.ebuild.