Its certain that these are only estimates, however, Bing has been growing in market share consistently for quite some time now. If memory serves correctly, they have had month over month improvement for at least the last 6 months, if not longer. Bing was around 6% when it "launched." Not sure exactly what Yahoo! was at then, but it didn't sum to 30. Initially, Bing grabbed share primarily from Yahoo! searches, but more recently it looks like more has come from Google (though it hasn't been that significant). Interestingly, Bing even grew in January when it was accused on "copying" results.

Point being, while doubting the accuracy of the numbers reported is reasonable, I find it much harder to dismiss a trend.