I think that OpenID is too confusing, until presented well to the user. I'd include a short piece of text on yuor registration/login page about OpenID - and make it clear to the user that they probably already use an OpenID-enabled service (such as Google Accounts or Yahoo! which actually tell you now). Try to present it as a way to speed up registration - less fields etc.

Right now, I'd still use a classical registration system, but show off OpenID as an alternative system and allow existing registrations to link OpenIDs. I still view it as an alternative, rather than a replacement.