Openstack in itself is indeed only a IaaS. PaaS would be the equivalent of having a pre-configured environment handed to you for consumption (think of Rackspace or Mirantis and their public cloud offering) negating the need for management of hardware or software infrastructure, and SaaS would be possible through Openstack... but Openstack is not SaaS. (a more precise example of SaaS would be something like Google Docs)