So that's what was missing! I knew something was disrupting my normal flow of analyzing search results... and now that you mention it I realize I usually look at the URL's, sometimes even before the article title.
Great work!
One more thing I think is necessary, you should clearly mention in the about page that you are feeding from Yahoo search result as well. It is fair to the user and to Yahoo.
Yahoo BOSS's terms do not require attribution though: http://developer.yahoo.com/search/boss/ . I don't think most users would care as much either; we hackers care a lot coz we're baffled with "how in the world did this one guy do this?!". (Even with BOSS being used for some results, this is still very impressive.)
Although it might help the service get more credibility by mentioning that Yahoo results are also used.
Definitely, because you have many more types of links than a normal search engine--some of them are requests for clarification. A URL tells the user "This is a 'hit' that will take you to a different website".