EVERYBODY in the SEO world tells you to optimize for google. But bing + yahoo now is half of google, according June 2010 Comscore data http://www.comscore.com/Press_Events/Press_Releases/2008/07/Top_US_Search_Engines
Bing + yahoo – 30%
And since august 2010, yahoo will use the Bing search engine.
Conclusion: if you want traffic, you better start optimize for Bing also. Is also much cheaper then in the crowded google optimisation world