Matt,
One other option might be to use a fax-via-email option with PP Image, either using an in-house fax server (we've used Solgenia's FACSys product for several customers) or potentially even using one of the Internet email-to-fax services.
In a couple cases, we're running the fax server in a vm as well, and using a small FoIP appliance to provide the actual analog fax ports. This allows us to retain full vmotion capability within the cluster.
This option will definitely cost more than a single attached fax modem, but then it depends on how important the VM option is to you...