Here’s another interesting report. Google provides some research/analysis on federated logins using technologies like OpenID:
Most websites who offer a user registration mechanism find that at least 30% of users who start the process fail to finish (if you include sending them an Email to verify their ownership of the Email address). The percentage success rate drops even further as websites ask for more information. One potential method for increasing the success rate is to support a federated login mechanism that automates most of the registration process and removes the need for users to remember a password for the website.
I’m a big fan of OpenID, personally, and have long wanted to do more with it (including allowing i-Proving comments with OpenID).