It's been sometime since yahoo have announce OpenID support, for the last few days, it is not open for registration.
Getting it is easy, just go to
http://openid.yahoo.com/Then select your identifier, which include your flickr account home page, as your identifier.
What make it cool, it it is able to use, flickr account as my OpenID identifier. Which make me easy to remember my identifier. Now that, yahoo support OpenID, I don't really need to register another account to use OpenID enabled sites