Seems like we can finally answer all the "what about Yahoo" questions in the affirmative: Yahoo goes on-line with their OpenID implementation. Mike Arrington calls it a "Massive Win for the Project" and writes:

The rumor last week was that Google (as well as Verisign and IBM) were mulling over the idea of joining the OpenID 2.0 single sign-on framework. But the real news comes today, as Yahoo and its roughly 250 million user IDs officially jump on the bandwagon. Today, there are only approximately 120 million valid OpenID accounts. In one move, Yahoo more than triples that number.

Welcome aboard, Yahoo! And thank you all within the OpenID Foundation and in the larger community making this happen. It took a lot of persistent work over many months.

Here is the press release.