The third-party Siri, before Apple purchased the company, worked on any iPhone running the version of iOS it targeted. After the purchase it was relaunched as an integrated OS feature for the 4S and no longer available for older iPhones. Jailbroken older phones could reenable the feature.

The 4S did come with noise-cancelling microphones and an extra chip to support this; I believe the official line is that this hardware was required for Siri to work with sufficient reliability.

Edit: here's an article about jailbroken Siri: http://www.cultofmac.com/133196/fully-functional-siri-arrive...