HN was using Clickpass (YC Summer 07), which was acquired and shuttered by Janrain early last year. Amusingly enough, Stack Overflow still has a (broken) Clickpass button on its login page.
If I recall correctly, Clickpass was a centralized OpenID proxy, whereas Persona is trying to bootstrap a fully decentralized protocol. We don't want to even be able to track you. :)
[Edit: Corrected YC cohort. Thanks wamatt!]