Siri is no longer exclusive to the new Apple iPhone 4S as it has been proven to work properly on an iPhone 4. According to SlashGear, developer Steven Troughton-Smith has achieved this impressive feat by jailbreaking an iPhone 4S and porting the proper files for Apple's servers to authenticate requests from the iPhone 4.
SlashGear - “It does require files from an iPhone 4S which aren’t ours to distribute, and it also requires a validation token from the iPhone 4S that has to be pulled live from a jailbroken iPhone 4S, and it’s about a 20-step process right now” - Steve Troughton-Smith, developer
When asked on a public release of the Siri hack, Troughton-Smith has stated that he had no interest in making it available for other users as his aim was to prove that Siri could work on the iPhone 4 and other devices.
Troughton-Smith also demonstrated that Siri could function on the iPod Touch although the user experience was marred by its hardware limitations. The microphone on the iPod Touch appeared to be more less sensitive than the iPhones in picking up voice commands, hence requiring a user to speak louder and more clearly.
Now that Siri is working fine on the iPhone 4, you have fewer reasons to get the iPhone 4S unless you want the faster dual-core processor and improved 8-megapixel camera.