This has always appeared to me as single sign on between two Sitefinity sites which is not at all what I want. I want to be able to do single sign on with other partner sites which are most likely not going to be Sitefinity sites. Most sites use SAML, Liberty Alliance, WS Federation or Shibboleth or now Adobe's ADEP. Sitefinity is using WS Federation so one would hope you could setup an SSO relationship between Sitefinity and another partner using WS Federation claims-based authentication. I would want to be able to set up this relationship with applications using SAML, Liberty Alliance, WS Federation, Shibboleth or ADEP not just WS Federation if that is possible.
I think SSO conversations never go far on the forum's and the blog's because it is always assumed that the only need for it would be to have SSO between Sitefinity sites and even then for administration never any focus on front end users registering to gain access to content.
For front end users I think social networking authentication SSO would be very useful.