This might be big news for the Semantic Web, or it might not mean much at all. In one way, this doesn't change very much - Yahoo! is very much niche as far as search engines are concerned (in the UK at least), MSN/Live has overtaken them in terms of referrals on the websites I work with (probably because of all the new installs of Windows which have that set as the default search provider, rather than because their search is better), but both are dwarfed by Google. In another way, it might be enough benefit to make people think it's worthwhile to start adding more semantic markup, which in turn might lead into a virtuous circle which quickly ramps up the amount of semantic web content available to a critical mass.