Yahoo and Google rank sites using different criteria, which is why their results are so wildly different.
In fact, I've recently been using Yahoo far more than Google since its results are often actually more useful than those returned from Google.
Without getting into the meat of it too much, some engines use keyword weights, some still like to see useful and appropriate metatags and some just read your page/site content as a visitor would and attempt to match its content to search queries.
The Yahoo system, although totally different to Google, sometimes produces better results. Not always though.
Personally I ignore search engines as much as possible. If I design a search engine sympathetic site and refresh its content regularly, if the layout and underlying cosing is good about the best chance it has after being published is to be effectively publicised by its owner through more mundane means (advertising, letterheads, business cards, general company stationary and so on).
Not too many businesses have or are prepared to spend the sort of cash a good Google ranking can cost these days, and a good Google ranking is not necessarily equal to a good Yahoo or Altavista rank.