Google is currently 5th for the search term "Buy Viagra" at Yahoo.co.uk There is an amusing article about this, but I am curious to know why you think this has happened? I will share my theory soon. Viagra jokes are also welcome
http://forums.digitalpoint.com/showthread.php?t=255198 Google is now using interstitial pages for those using Google's url redirection script. These are used to cloak spammy pages, etc. - using Google's weight. I bet that link went to a pharmacy site or that interstitial, and not the Google homepage. The interstitial publishes a 302 redirect to the url being used, which seems to be vulnerable to a hijack, and Yahoo picked up on it, I think. Yahoo is good at fixing these things, they are rare and temporary.