When you hit the search hardware button on Windows Phone 7, the default search engine Bing is loaded. If you are looking to get other popular search engines, then you can try out IE Search Switcher. It is a tool for quickly changing the search engine used when you hit the hardware Search button from within IE. The initial release allows you to use the search provider that comes configured on your phone, or the built-in Bing app. If you’ve manually added any more providers, you can switch between them too.

The application is developed by XDA Developers forum member and you can get more details here.