I assume you saw Allen's response on the OpenID list. Just incase, I have included his response.
-- Allen Tom atom at yahoo-inc.com Tue Jan 13 10:41:44 PST 2009
We're really happy to see RPs accept Flickr OpenIDs, because there's a lot of interesting things that RPs can potentially do with Flickr identities. In particular, a user's Flickr Profile and Photos pages contain plenty of interesting microformats that could be used by RPs to personalize the experience for Flickr users.
Currently, Flickr users must explicitly enable their Flickr Photos URL to be used as an OpenID. To do this, you can go to http://openid.yahoo.com and click the big "Get Started" button. After signing in with the Yahoo ID that is associated with the Flickr account, you'll be able enable your Flickr Photos URL as an OpenID by expanding the "Show Customization Options" arrow at the bottom of the screen.
Once you've enabled your Flickr Photos URL as an OpenID, you'll be able to type in your Flickr Photos URL onto an RP site and have it returned in the OpenID assertion. You can also just type in "flickr.com" and use directed identity.
If you have not enabled your Flickr Photos URL as an OpenID, we will return the default Yahoo OpenID (the ugly machine generated hashed identifier) in the assertion.
After reading this, you're probably wondering why this is so convoluted. When we launched our OpenID service a year ago, we were required to issue machine generated OpenIDs (the ugly hashed ones) to users by default, unless they explicitly asked for a personalized identifier. Our lawyers also insisted that all Yahoo/Flickr users who wanted to use their account as an OpenID explicitly enable their account for OpenID and agree to a new Terms of Service. The whole User Experience (UX) of enabling an account for OpenID, agreeing to a ToS, and then selecting a personalized identifier proved to be a horrendous UX with very high dropoff rates, which we formally studied, documented, and released to the OpenID Community here: http://developer.yahoo.com/openid/bestpractices.html
We can probably optimize the experience a bit by changing the UX flow to ask the user to enable their Flickr account as an OpenID when the authentication request contains a flickr.com URL.
An alternative approach (and probably better) would be to use Attribute Exchange to share the Flickr Photos URL with the RP, and to keep the default Yahoo OpenID identifier. There's even a Flickr URL attribute defined in the official AX schema: http://www.axschema.org/types/
If you have any more questions or feedback regarding Flickr OpenIDs, please don't hesitate to contact me directly, or on this list.