First Look: Using iPhone 4S with Siri voice assistant (with videos)
Siri outside of iPhone 4S
It's not clear how much of Siri's exclusive availability on the iPhone 4S is due to its far more powerful processor, and how much is simply Apple reserving the feature as an incentive to upgrade.
In previous iPhone hardware releases, Apple similarly reserved video capture as an exclusive of the iPhone 3GS, and made FaceTime exclusive to iPhone 4. While those features have been hacked to work on earlier models, they don't run nearly as well on the less powerful hardware.
Were Apple to make Siri available on the iPhone 4, it would likely work half as fast and half as well, likely tainting the experience of the majority of those able to immediately try it out. On iPhone 4S, the new feature works brilliantly, although not always flawlessly.
Apart from being far more sophisticated in what it can do, Siri also has a significant limitation over the Voice Control it replaces on the 4S: it only works if you have a network connection. In Airplane Mode, Siri won't work at all. The old Voice Control will, but its commands (and accuracy) are so limited that it isn't likely to get used that much.
Siri promises to change how often people use voice command by making it far more likely to work as expected, and to do functions that are valuable enough to remember how to use it. At some point, Apple will likely bring Siri to the Mac along with iOS 5's new iMessage chat, just as it did over the last year with FaceTime in iOS 4.
Siri is also likely to make its way to iPad 2, which is certainly powerful enough to run the software. It's less certain that Siri will be adapted to last year's iPhone 4 or the current crop of iPod touch models, none of which use the same speedy A5 chip that power the iPad 2 and iPhone 4S.