I just read about an interesting report sponsored by one of the major meta-search engines Dogpile. 

They analysed 780,000 first page results across the four major search engines (Google, Yahoo, MSN and Ask) and found that less than 1% (0.6% to be exact) overlapped across all four. 

Here's some more facts from the report:

• 88.3 percent of the results were unique to one search engine.
• 8.9 percent of total results were shared by any two search engines.
• 2.2 percent percent of total results were shared by three search engines.
• 0.6 percent of total results were shared by the top four search engines.

Source: MarketingCharts

If the search engine's major quest is relevance, then you would assume that the top "most relevant" results would consist of a finite set of pages and that these would be shared or at least overlap more across the major engines.    Well it just goes to show that are always opportunities for your site on each of the engines to help drive qualified traffic to your site.