Just a few hours ago Yahoo! announced that it has fully moved from using its own search engine to showing search results powered by Microsoft's Bing.
What does this mean for the search engine optimisation market? Having your website rank well in Bing will be more valuable than before (since ranking well in Bing will now mean automatically ranking well in Yahoo!). For United States businesses the change is only minor though, as Google has the lion's share of the search engine market in United States, with over 93% of searches performed through Google, as reported by Hitwise (24th July 2010).
In comparison Bing receives just over 3-4% of search traffic in United States with Yahoo!'s share being just over 2%. So while the combined traffic from these search engines can be meaningful globally (especially in the US where Yahoo!'s share is above 14% and Bing receives around 10% of the traffic), in United States for the time being if you're doing SEO you should still fully focus your efforts on Google.