t looks like Bing will not only be your default search option for Windows Phone 7, but Microsoft's home-grown search engine will be the only one used for the forthcoming mobile OS slated to launch in time for the holiday shopping season. Microsoft's senior product manager Greg Sullivan said in an interview with Pocket-Lint that “the search engine has been heavily integrated into the OS, so it would be hard to offer an alternative.”
With Windows Phone 7, carriers and manufacturers can possibly add a second search engine via download, but unlike other platforms Microsoft won't be making those choices available by default on the OS. According to the interview, "Microsoft would allow other search engines to create an app, or even, presumably, a hub, it has confirmed that it will not be letting any other company (read Google), be the default search engine for the new phone operating system."
One of the reasons for the limitation and Microsoft's explanation for the "heavily integrated" component of the OS is the integration of voice control and search app TellMe. TellMe will allow users to control their phone and search the net with their voice and will compete against Android's and iOS's voice offerings.