This means, if you ask Siri a question for which she needs to visit the web — either by asking her a question she doesn’t know the answer to or by asking her to search the web directly — the results you get will be from Google, not Bing.
According to Apple, this is to make searches more consistent across all its devices. Apple told TechCrunch:
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.
Apple is rolling the change out today globally. Web image results will still come from Bing, apparently, but video results will now come from YouTube.
Apple and Google both declined further comment.