A bad case of NIH

Client side SSL keys is a specific authentication method.

OpenID is not an authentication method but OpenID providers need one.

So the two are orthogonal.

All that OpenID specifies is a way for you to take a URI and find out whether the person who
gave it to you has authority over that URI.

It doesn't specify how that authority is determined. For some OpenID providers the answer is
that everyone has authority (this is somewhat useless but proves a point). Some OpenIDs are
shared by a few people, each of which has a different "password" to prove their authority but
is indistinguishable to the OpenID relying party. LiveJournal's provider says that each
LiveJournal user has authority over a URI associated with their blog. If you own a DNS domain
you can use software to create an OpenID provider running on your own machine which does
purely local authentication, or you could use your domain to just publish a web page with a
link tag that delegates your OpenID to another provider (thus you can switch providers without
losing your identity). Some newer providers give each person an SSL cert, and they can use
that to assert authority over a range of arbitrary OpenIDs so that they can use more than one
identity. They just type the provider's URI in when asked for an OpenID, so no need to
remember whether they signed up for this service in their persona as "Arnold Harrington" the
40 year old respected accountant or "Gary Giblets" amateur comedian popular in working men's
clubs, their OpenID provider remembers that for them, and if they sign up to a new service it
offers to provide these existing identities or a new one of their creation.