As posted on their website http://developer.yahoo.com/openid

Yahoo! Supports OpenID 2.0!
OpenID makes it easy for you to deliver a simplified login experience for your users. Rather than having to create a unique user name and password for your website, users can now use an existing user name and password that they have created elsewhere. Of course, this is true only if “elsewhere” is a participating supplier of OpenIDs.

At Yahoo!, we acknowledge that simplifying the user login experience across the Internet benefits everyone. That’s why we are a huge supporter of OpenID. By enabling our 248 million registered users as OpenID accounts, we hope that gives you a lot of reasons to support OpenID too.