[drdaeman]

I know only two cases Google ever allowing to use externally-hosted identities to interoperate with their services: GTalk and Wave, both having such possibility only due to XMPP as their base.

If I understand it correctly, Buzz has interoperability by some means, but not in area of user identities - only Google users may use Buzz.

I sincerely hope G+ will have strong federation support someday. But for now, such articles, telling that "G+ is about federation" (while there's even no API yet, or I missed something?) are over-optimistic and sound more like an advertisement.