After iTunes Match is launched, a software upgrade could bring Siri to some older devices (if this is indeed the plan for Apple). It would make sense for the company to start upgrades with the iPhone 4, with the iPad and iPod touch updated soon after. Although I wouldn't be entirely surprised if Apple refuses to release Siri for older devices, the continuing availability of the iPhone 3GS indicates that Apple is pleased to cater to the market of second and third wave adopters who may not have the means or desire to buy into the bleeding edge.