http://www.netmarketshare.com/report.aspx?qprid=5&qpaf=&... (global, have to pay for US only data).

And as already mentioned, Comscore shows Google at an all time high in the US.

Bottom line, these types of things are very hard to measure, so take every statistic with a grain of salt. The only thing we really know is that Google is the king and Bing is slowly growing, mostly taking marketshare from Yahoo. Anecdotally, most people who run websites are seeing barely any traffic from Bing, and on a global scale, Bing is basically a non-factor.