It's become clear that Google+ is precisely what it looks like: Google doing for Human Identity what Google did for the Web. Google is developing the value of the information we put into the internet (for our mutual benefit). Clearly the social network is the home of Human Identity on the internet.
Identity is critical in the world of unrefined information -- information not carefully compiled according to a disciplined logic, which is most everything that's not Science Data.
If identity were logically crisp (globally precise), then the identity of authorship and the networks of association, sharing and collaboration would be refined information.
The problem with simple identity services like Google Accounts is that they're mechanical. I have a half dozen Google Accounts that I employ for reasons that are a combination of project structure purposes and Google quota subversion. This situation dilutes the value of Identity in Google's data networks. Before Plus, Google had a strong sense of projects (code, blog, video, etc), but a poor sense of Identity.
This is ultimately what they mean by "who are you". If a DNA sequence was a number, that number would be generally the same idea (while cloning is fiction the number is unique).
There's a continuum from poor mechanical identity to crisp, refined identity information. Moving the value of Human Identity information up this range into higher values requires open solutions that permit identity to be applied. The value is in the application, otherwise it is only info economic theory.