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. §
Yahoo! still does not get it though. Its nowhere near Gmail in simplicity and easy of use! Naah. I guess it just proves that there’s no point copying a competitor unless you also understand what it is that really draws users towards their product. Yahoo! Mail Beta is got lots of AJAX, but none of the simplicity and non-clutter of Gmail. And its got long ads on the right side of the window.
In its defense, Yahoo! Mail Beta is much much much better than Microsoft’s Live Mail (Kahuna). Microsoft’s offering has to be the worst of the lot!