Yes, it has finally happened. According to a statement released by Apple today, now when you ask Siri a question and it falls back on a web search, the default search engine used will be Google instead of Microsoft's Bing. The company had the following to say about the change, citing the consistency of search results as the reason behind it:
Switching to Google as the web search provider for Siri, Search within iOS and Spotlight on Mac will allow these services to have a consistent web search experience with the default in Safari. We have strong relationships with Google and Microsoft and remain committed to delivering the best user experience possible.
TechCrunch further explained the ins and outs of Siri's functionality post-Bing:
The search results include regular 'web links' as well as video results. Web image results from Siri will still come from Bing, for now. Bing has had more than solid image results for some time now so that makes some sense. If you use Siri to search your own photos, it will, of course, use your own library instead. Interestingly, video results will come directly from YouTube.
This is all good news for Apple users who continually find Bing search results lacking. It can be frustrating to begin with when Siri can't answer your queries, but to be served subpar search results afterward is a bit like adding insult to injury.
You can find more details about the change in TechCrunch's article: