A while ago I pointed that more and more, Yahoo is relying on Google for it’s search results. Tim Parkin pointed out that a Yahoo listing will give a higher Google ranking, so from that point of view being on Yahoo does still have some importance.
In fact, the reason a Yahoo listing may give you a better ranking on Google is not due to some dodgy nepotism, but because Yahoo itself is a highly ranked site. Any link from such a site will in turn give a good Google rating to your site. You can see the top rated sites on Google by searching for http://. At the time of writing, the top 5 are Yahoo, Google, Microsoft, Adobe, Alta Vista, with FT, and Wired, bringing up 99 and 100.