Getting Yahoo! Mail Beta
To get Yahoo Mail Beta, just switch your content preference to Germany, France or UK. Then you will be asked if you want to join the beta when you log into your Yahoo Mail. Say yes, and join the beta. Then from the options menu, change your content preference what it was before. Then go to Yahoo Mail again. You should see Yahoo Beta. If you don't see it, go to options and click "Try Beta" button. That's all.
Somehow, UK wasn't available, so I had to max out my linguistic skills to wade through Yahoo! German :-).
Willkommen zu der Beta-Version des neuen Yahoo! Mail!
I'm on Yahoo! mail beta now. The new Yahoo! mail is yet far from finish though. The interface looks slick, but loads a bit slowly, and I even got one alert window (usually placed for debugging etc). So it's alright that yahoo! is rolling it out slowly. The Outlook-ish three-pane view is the highlight of the new service, and it works perfectly on Firefox.