We're not kidding when we say that Apple and Google have developed a close relationship. When you tap the iPhone Safari address bar, a screen opens offering both a keyboard and a search field. By default, this field is tied to Google. When empty it says Google in dim gray letters. When you start to type in text the Go button changes to Google.
But there's a way around that.
You can use Yahoo rather than Google as your search provider. From your Home screen, go to Settings > Safari. The first setting lets you choose whether to use Google or Yahoo as your search engine. When set to Yahoo, both that gray text and the search button change from Google to Yahoo, as well as your searches.
I'm not entirely sure why people might prefer Yahoo to Google, still it's good to have a choice.