That's correct Adam. I was in the admittedly bad habit of calling an external SSO on 5.5 a PSC since I am in the process of upgrading 2 vCenters with one SSO environment. After discussions with my TAM we were referred to Ryan Johnson's blog post
He has some great slides as well at http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2033620
After discussing my environment with Ryan he suggested making our SSO domains for both vCenters external in preparation of upgrading to 6.1. So in my mind I had already made the jump to calling the external SSO servers PSCs.
Thanks for the clarification.