Steve Rubel's Micro Persuasion wrote Yahoo Testing Blogs and RSS Search. Basically, the test site was at http://test.rss.search.yahoo.com/ which now redirects you to the advanced search page. Two small screen shots were captured by Steve, one of the homepage and the other of a search results page. There is a really large image at Flickr.
For some reasons, I haven't found any threads at the major SEM forums. So I started a thread at Search Engine Watch Forums.