The new Yahoo homepage, available at http://www.yahoo.com/preview supports only Internet Explorer and Firefox. Opera and Safari's users can try the new homepage at http://www.yahoo.com/beta.
There are small rendering issues in Opera 8.5, but Opera 9 fully supports the page.
I think webmasters (and especially big companies' webmasters) shouldn't launch a page or a site until every browser can render it. If a browser doesn't support all the Ajax features or CSS 2.0 or other standards, that elements of the web page should degrade gracefully.