There is a search form specifically for plugins here. These are the results when I search for "Add From Server", (quotes included). The page shows nothing about yahoo being the search engine used to display the results. I even checked the page source.
At the bottom of the page, there is a link that says, 'You may also try your search at Google'. This brings up the plugin as the first item on the list.
When I type "Add From Server" in the search form in the main wordpress.org header, it replies 'We Couldn't find anything.' Yes, this search says it's powered by Yahoo, but when I go to the link on that page for 'Yahoo Web Search' and type in "Add from server" again it shows the plugin as the first result.
If I go to Yahoo.com directly and similarly type in "Add From Server", I get the plugin again as the first item.
Code view for the general search form:
<form action="http://wordpress.org/search/do-search.php" method="get" id="head-search">
Code view for the plugin-specific search form:
<form action="http://wordpress.org/extend/plugins/search.php" method="get" id="plugins-search">
Neither of these forms goes to Yahoo's search api. There is no way the plugin search function or the general search function on wordpress.org is really using Yahoo web search. Either that, or both forms are malformed.
My suggestion has been from the start - either fix the wordpress.org plugin search so that it actually functions, or just use google's api. The current search is a waste of space in the header and the plugin page. Period.