Sadly, Yahoo! seem to have followed Google by weighting Wikipedia pages higher than ever before. So now we see appalling Wikipedia articles outranking far superior pages that used to be above Wiki in Y!
Don't forget that Wikipedia is a high-traffic site with pages that tend to have a lot of (good) backlinks and (mostly) good content, so I think that if you're seeing their pages ranked highly it's probably a result of the algorithm, not necessarily of some type of manually assigned bonus. From that perspective, if more of their pages are showing up in the Yahoo! search results, it may actually be a sign that the quality of those results is improving, that they are getting better at providing authoritative results.