John Gruber in his review of the iPhone 4S:

In a sense, Siri is like a second interface to iOS. The first interface is the app interface. Launch, tap, drag, slide. The Siri interface is a different world. As stated above, this new interface is in many ways the opposite of the regular one — open-ended and implicit instead of narrowly defined and explicit. I don’t mean to imply that Siri doesn’t fit in or feel right at home — it does. But Siri is indicative of an AI-focused ambition that Apple hasn’t shown since before Steve Jobs returned to the company. Prior to Siri, iOS struck me being designed to make it easy for us to do things. Siri is designed to do things for us.

“A Second Interface to iOS”