Here’s some step by step directions to force your account to switch over to the Yahoo! Mail beta. It worked for me even after I switched it back to Yahoo! US. It’s very Ajax-y, but I am sticking with Gmail.
* log in to Yahoo Mail
* click Options
* select Account information from the left panel
* go to Member Information, General Preferences, Preferred Content
* select, for example, Yahoo UK
* click Finished
* go to Yahoo Mail
* you’ll see a page that says “It’s the New Yahoo! Mail Beta… and you’re invited.”
* click on “Try Beta Now”.
You can then switch back to Yahoo! US.
Update: You can also just click here.