new Yahoo mail appears to be on Beta testing. I have it, but only because it asked me when I logged in a few days ago. Now everytime I log in, it's the new version. I suspect they'll iron out the creases, then launch it to everyone. They may have needed x number of users to try the beta version, so collected that many, then stopped the option to try it. Lucky me. I'm just guessing though.
It's quite good, but I've not had a good look around it yet.