I've been experimenting with OpenID for a couple of years now. My first OpenID was with JanRain's "MyOpenID" service as a delegate for my homepage. At the time, the service was amazing. They supported client certs, multiple profiles... the works.
Now it saddens me to report that JanRain's MyOpenID is effectively dead, as far as anyone can tell. For over a year, there has been a serious malfunction with the client certificate feature, and no indication from JanRain that they are even aware of the problem. I have tried to contact them via multiple email addresses (all of which bounced my message) and via their contact forms. On both their official UserVoice thread and a GetSatisfaction thread, many users report the same issue, but no JanRain representatives weigh in.
For a replacement service, I recommend switching to Clavid. And remember, delegation is your friend.