Although voice recognition for personal computers has been available in some form for almost 20 years, and it's been available in limited fashion for mobile devices for a while, the implementation of Siri in iOS 5 has been a matter of building voice control into the foundation of the operating system. That's going to change the very nature of the way we use voice to control our devices.
The dirty little secret of voice command is that it's very difficult to do properly. Twenty years ago, when I first began to look into it as an end-user solution, it was prohibitively expensive. In the intervening years, the software's become far better, and the cost of hardware has become all but insignificant.
I'm just starting to use Siri, and it's almost as useful as it is entertaining. Although the mispronunciations (for example, "Lou-EEE-ville") and the exaggerated politeness ("Ever so sorry, P. J.... I'm not allowed to do that.") may not rise to level of joke fodder that the Newton provided, we'll start seeing more and more voice recognition gags in the sitcoms of the future.
The reason why I'm sold on Siri isn't a matter of what it can do now, but what it will enable in the years to come. When voice control features are a fundamental part of the OS layer, instead of being an application or a bolt-on feature, it becomes much easier to develop applications that use such features.
Siri today is much like the microwave oven that was installed in my grandparents' kitchen in the 1950s. At the time, it wasn't good for much more than parlor tricks, because there weren't any cookbooks or specially-designed food for it. Today, of course, you can pick up a microwave oven that's safer, more useful, and far cheaper.
Siri is a milestone in voice recognition, in providing a platform for such services that's built into the device. That's going to prove irresistible to developers who would love to offer these capabilities but don't have the leverage to negotiate an agreement with Dragon and other holders of related IP. When Apple does open up Siri's APIs - which may not be for a while - it's going to give commercial and corporate developers a whole new playground for frolicking; eventually, the gee-whiz will wear off and we'll take voice control for granted, as we do our microwave ovens.