Microsoft already displays Twitter results for queries placed on its Bing search engine, although they are displayed on a separate page that is not integrated directly into the main search results.
One crucial difference between Google's and Yahoo's approaches is that Yahoo has not cut a deal with Twitter for access to the 'firehose', an automated feed of data from Twitter. Instead, it is using Twitter's public API and adding its own algorithms to work out which tweets are most relevant to the query.
For more, read "Yahoo joins the real-time search parade" on CNET News.