That depends on who the site's audience are and how the login UI's done. Fairly tech-savvy visitors certainly do understand OpenID and alike. At least, I strongly believe so. Well, more casual audience probably don't.
However, my bank uses X.509 certificates to authenticate users over Internet. Out of curiosity, I've asked bank employee once and was told that most users do manage their certificates just fine, without any issues. Obviously, bank provides short and simple manuals, which explain how to generate a certificate request, obtain a signed certificate (by visiting bank in person), authenticate and renew expiring certs.
(My bank's not using in-browser PKI due to legal reasons and provides a small piece of software wrapping around the browser, but that doesn't really matter.)