Infoworld reports this morning that Yahoo appears close to becoming an identity provider for OpenID. This, of course, is the next step in a full implementation. You can already use OpenID to log into Yahoo properties like Flickr, for instance.
I've been using OpenID as a single sign-on for a while, for sites that allow it, via the OpenID plug-in for WordPress. It's a valuable and useful tool, hampered by the fact that there simply isn't enough support for it. With additional support from Yahoo, it looks as if momentum might be shifting finally.