Everyone's saying Siri won't come to older iPhones, but this report may be premature. It's based on an email Michael Steeber received from Apple support about a bug in Siri. The customer service rep was told by Apple's engineering team that "Siri only works on iPhone 4S and we currently have no plans to support older devices."
Most people are focusing on the "have no plans" part of the response and overlooking the key word "currently". Yes, right now, Apple doesn't have plans to expand Siri, but these plans could change next month. It's not like Apple hasn't said one thing and done another before - remember the iPod, which was deemed to have a screen too small for video?
Looking beyond words, there are other signs that suggest Siri won't be exclusive to the iPhone 4S. First and foremost, there's no technological reason why Siri won't work on older handsets. The iOS hacking community has managed to get Siri running, as a proof of concept, on the iPhone 3GS, iPhone 4 and both iPad models. The major hurdle these developers had to overcome was Apple limiting Siri requests to the iPhone 4S. Siri can and does run on older hardware. Apple's just not allowing it, at least not yet.
Apple may be proceeding slowly with Siri. It's a beta product and still undergoing refinement. Just look at Apple's Siri FAQ page and all the new languages that are being added. Apple's smart enough not to swamp their system by rolling out Siri to millions of iPhone 4 handsets at the same time they are still adding features.
Apple customers just need to be patient and wait. Wait for Apple to flesh out the service and get it firmly established on the iPhone 4S. By that time, Apple will be done using the voice assistant to sell their flagship handset and will be ready to bring Siri to the iPhone 4 and possibly other older iOS devices.