I was almost shocked to see they more than doubled the price for the Yahoo Mail Plus, for just naming it something else. I decided I wouldn't pay them, and started developing a fix (a Firefox add-on), which is now hosted and getting very good response from people. Just thought of sharing that with you guys.
This is the Firefox add-on:
Yahoo Mail Hide Ad Panel
If you don't have Firefox browser, you can download it from here:
http://www.mozilla.org/en-US/firefox/new/