It's a question I haven't been able to answer for friends who've been dismissing the decision as nothing more than a marketing tactic to move more iPhone 4Ss. My reply that Apple didn't really help moving iPhone 4Ss and that they will sell millions of them anyway didn't help much. They're suspicious of Apple's motives here, as well they should be as of any large corporation. But I refuse to believe that's how Apple rolls. My other explanations went something like this: Siri needs more RAM. iPhone 4S should have 1GB of RAM compared to the iPhone 4's 512MB. Well, turns out the 4S also has 512MB and not more. So that's not the reason. Siri's app store app is proof that Siri *could* work on previous iPhone models, so it seems hypocritical that Apple would then claim it will only work on the 4S. But there's a big difference here the way I see it. App Store Siri was an app. iPhone 4S Siri is deeply, deeply integrated into iOS 5. So deeply, in fact, that I'm sure they've optimized the hell out of it interlinking with all the services that it hooks into, like Calendar, dictation for everywhere there's a keyboard, search on Wolfram Alpha and Yelp, Reminders, etc. It's a whole different kind of operation than Siri running as an app, which in comparison provided drastically less functionality. At this level of integration and with the optimization it is likely to require for everything to work smoothly, I can absolutely see why the feature is limited to the 4S. Ok, ok, so why is Apple taking the Siri app, that runs perfectly fine on previous devices, out of the App Store? Well, simple. Siri, the company, is no more. It's been swallowed by Apple. They have no plans to maintain the app and provide further updates. They have rolled its functionality into iOS 5 on the iPhone 4S. With no further development, there is no point in keeping it around on the App Store. The last two "answers" or explanations represent my current stance on the topic. The former makes a lot of sense to me. The latter does too, although I could see Apple keeping the app in the App Store, letting it gather dust, and then have distraught users of the app automatically look to the iPhone 4S and how there, the feature works beautifully. But that seems evil because it would mean using a potentially deteriorating user experience as a marketing tool. And again, that's just not how Apple rolls. Comments?