OpenID is NOT about physical identity, bare in mind. It's all about an identity on the Internet. An OpenID identity is just a URL. You can have multiple URLs, yep, you can have multiple identities. All OpenID does is provide a way to prove that you own a URL (identity).
The idea of claiming an URL is not new, anyway. Sites like Technorati and BlogShares use this technique for quite some time. But probably the novelty of OpenID is that it bring that claims to an identity system.