Have you done any testing to see how many users you lose by doing this? There is, after all, a percentage of your users who will see your "login with Yahoo" message and not understand what you mean, then leave when they can't find a way to register.

You seem to think that number would be low. Experience with users & registration leads me to believe that it will be quite high. I personally don't plan to implement openID, so I can't do any testing. I'd be curious to see what your numbers say.