How does a public key identify the person using it? It doesn’t.
palmer explains it better above:
http://www.freerepublic.com/focus/news/3468215/posts?page=45#45
In my example, Alice identifies herself with the public key. It true, there is no information in the public key from which you can derive Alice's identity, but that does not stop Alice from doing so herself.
Perhaps palmer will return and shed some more light on this.