When Apple showed the world Siri, I believe they showed us the next major man-to-machine user interface.
The idea of talking to computers is nothing new. It has, of course, been featured in sci-fi novels, movies and TV shows for years now. Even software itself has shown shades of voice input as the next user input for over 10 years. The challenge had always been bringing it to the mass market. This is what Apple plans to do with Siri on the iPhone 4S.