I've always wondered myself where their market share numbers come from. My guess is they have heavy default homepage numbers that drives traffic and probably pretty good numbers from the my.yahoo stuff. But as far as search, I just don't see it. I have a general business term that I'm #1 at Yahoo and #2 at Google but Google historically generates 3x more traffic than Yahoo.
One possible explanation is that it is more related to the targeted user of a site (mine are more business-related). It seems Yahoo tends to be more consumer-oriented than business-oriented.