According to one report, Yahoo is presently in negotiations with Apple to replace the Google-driven Search box in Apple's Web Browser (Safari) with one powered by Yahoo's search engine.
Apple's web browser offers a built in search box in the top right corner window which allows users to quickly search the web. Since Safari's introduction, the search box has used Google's popular search engine to provide results.
Google's prominent placement is presumably provided for convenience rather than any specific financial deal between the two companies.