To be sure, some (most?) of Microsoft's gains have come with an expense: The company is buying up toolbar deals to become the default search engine for more users — less-valuable, paid traffic that Yahoo seems happy to give up. And Microsoft has spent a lot of money advertising Bing.
But there's no doubt that Yahoo's declining search business, long term, is bad news for the company. Especially because its deal to farm out its search technology to Bing will only generate revenue for searches conducted through Yahoo, not through Bing, even though Yahoo is selling the ads on Bing.