Eric Sachs, Product Manager for Security at Google, has published an excellent document detailing the results of a Google-internal usability study on federated login. It is written largely from a protocol-neutral perspective, but has immediate applicability to OpenID.

He pays particular attention to how not to confuse users who already have an account at a site and are not technically inclined.

One aspect he doesn't address is whether users got confused by the redirect dance between RP and IdP. But this is the most detailed, most thoughtful description of the real-world usability issues of OpenID and friends that I have seen so far. Way to go, Eric!