Please consider adding certificate-based authentication to Stack Exchange as another form of authentication. I believe this method of authentication is probably closest to what Jeff describesas a good solution in his blog.
It sure seems like this should be pretty easy to implement. It should be as trustworthy for proving the identity of someone as trusting 3rd party identity providers, or simple cookies for authentication. You already support multiple authentication providers adding another seems like it would be easy.
Certificate based authentication isn't very common at the moment, but has many of the properties Jeff seemed to desire in his blog. The only way it might become common if someone big actually does it, and does it well. Providing certificate based authentication as on option on a platform many developers use seems like a good way to let people test it out and see how easy it could work.
Doing the certificate authentication should be easy. Just setup a page, which requires a certificate. If you have never seen the public certificate that is offered by the browser before, then ask the user if they wish to associate the certificate with an existing account or create a new one, store the certificate for future logins to that account. If you have seen the public cert before, then look up the account details and log the user in. Please be sure to allow a user to have a few certificates stored, and give them an easy way to manage the certs associated with their account.
I would suggest that you should not care about what CA the certificate is signed by. Just accept any certificate signed by any CA, or self-signed. I don't think the CAs really offer much useful for stacexchange. Just trust that the first use of a certificate is from the user it belongs to.