I get the impression that Yahoo has some kind of thing favoring Wikipedia, but that Google doesn't. Wikipedia is okay on things that aren't controversial, but in some areas there are a lot of guys trying to chisel facts to suit some kind of conspiracy theory or urban legend, making Wikipedia dangerously unreliable. With regular websites, you can judge the authority to some extent. With Wikipedia, who or what lies behind the words is secret. I hope Google penalises it somewhat if it starts to creep to the top of too many searches.