Though Microsoft's Bing search engines isn’t available inside the iPhone, but the Redmond company’ve released a new software development kit (SDK) to help third-party iPhone and Mac developers use the search engine inside their applications. The Bing iPhone and Mac SDK is open source under Microsoft Public License (MS-PL), and provides:

• ability to easily query Bing from within your Cocoa or Cocoa Touch application.
• Perform both synchronous and asynchronous queries.
• Search Bing for Web, Image, Video, News, and Phonebook results.

Download from CodePlex