According to latest Hitwise release "In the first five days of its search alliance, Bing and Yahoo! combined to hold 24.15% market share for the week that ended on August 28th — reflecting five days of combined search results. Individually for those five days, Yahoo held 14.28% and Bing 9.87% of all U.S. searches."
Google accounted 71.59% of all U.S. searches conducted in the four weeks ending Aug. 28, 2010.
Yahoo! and Bing announced that the transition to using Bing-powered organic results on Yahoo was completed on Augt 24th.