In Simple Addressing for the Web, Blaine Cook puts forward the idea of using email addresses as identifiers. Some very good ideas here. I do agree that email addresses are a simple, established addressing scheme to build on, but I do wonder about giving the power of providing identity to Google, Yahoo, Microsoft, etc.
I have two reasons for really liking OpenID - I can manage logins from one place, and I can use my own site as a delegate. There are heaps of OpenID providers. Could we have the same things using email addresses? Other options include using the browser to manage your identity. This might run into problems with people that don't tend to use a single computer. I also like the idea of combining a bunch of relatively new standards to map emails to URLS. This still has the issue of who is the provider, and how easy is it to choose. It's good to see the identity movement gaining some steam in the form of support from Google.