iOS 7 still uses Google for the default search engine in Apple's web browser, Safari, but Bing is the new default for Apple's virtual digital assistant, Siri. Yes, just like Open Table is the partner for reservations, and Wikipedia for general information, Bing and Microsoft worked out a deal with Apple for Siri search. So, while you can't change that, you can tell Siri to search Google (or Yahoo!) instead. Here's how:
1- Hold down the Home button to activate Siri.
2- Tell Siri to "Google" or "Search Google" and what you want to search for. For example: "Google 'Astonishing X-Men'"
3- Watch those Google results come flying in!
That's it! Yes, you do have to say "Google" every time, but it's not much more mental of vocal work that saying "search" without specifying an alternate search engine.
Give it a try and let me know how it works for you!