Interesting article. Essentially, research seems to show that the results don't overlap as much as one might think between the various search engines, even on popular search terms.
One quote I found particularly interesting:
Lim says that they've made a number of interesting observations in using the tool. For example, the majority of sites that are ranked highly in Google rank poorly in Yahoo, and vice-versa. This suggests that webmasters optimizing for Google alone may be missing significant traffic from other search engines.